Map from Google - Australia

1- Arid climate
2- Mediterranean climate (Perth, Adelaide)
3- Tropical climate (Darwin, Brisbane)
4- South-east (Canberra, Sydney, Melbourne)
When to go

Australia, a huge country of more than 7.5 million square kilometres, crossed by the Tropic of Capricorn, has largely an arid climate, desert or semi-desert, except in the extreme north which is tropical with a rainy and a dry season, and the southern coasts which have a more temperate climate, oceanic or Mediterranean. Even non-desert areas are subject to sudden warming, caused by torrid winds coming from the desert, sometimes accompanied by sand.
Being in the Southern Hemisphere, Australia has reversed seasons compared with Europe or North America.

Area with an arid climate
In this vast area called generically "Outback", the climate is arid, since the annual precipitation is below 200 millimetres, so the climate is desert (inside the burgundy line), or between 200 and 400 millimetres, so the climate is semi-desert (within the area in blue). In Australia there are several deserts, such as Great Sandy Desert, Gibson Desert, Great Victoria Desert and Simpson Desert. In this area, there are no large cities and the population density is low.
In the semi-desert area, between the burgundy and blue lines, the rains fall in the form of downpour or thunderstorm in the hottest period, at least in the north-central part, while in the southernmost part the rains occur mostly in winter. In the innermost area, the rains are very rare and sporadic, however, every now and then a thunderstorm may occur, most likely in summer.
Temperatures vary depending on areas: winter is milder in the northern part, while summer is hot almost everywhere, except in the southernmost part and in the short western coastal stretch. In the north-west coast of Western Australia (see Eighty Mile Beach), the heat is intense even along the coast, for example in Port Headland the average daily temperature in January is 31.5 °C.
Here are the average temperatures in Port Hedland.
Average temperatures - Port Hedland
Port HedlandJanFebMarAprMayJunJulAugSepOctNovDec
Min (°C)262525211714121315182124
Max (°C)373637353128272932353637

In this coastal area the rainfall is scarce, since it amounts to about 300 mm per year. Here is the average precipitation.
Average precipitation - Port Hedland
Port HedlandJanFebMarAprMayJunJulAugSepOctNovDecYear
Prec. (mm)65100452025208511215310

The sea temperature is warm all year round in Eighty Mile Beach, as you can see from the temperature of the sea at Port Hedland, which is located south of the beach, and therefore is slightly cooler in winter. Here the heat is intense from October to April.
Sea temperature - Port Hedland
Port HedlandJanFebMarAprMayJunJulAugSepOctNovDec
Sea (°C)292930292726242424252628

The maximum temperature exceeds 35 °C from December to February in Oodnadatta (in the state of South Australia), where 180 mm of rain per year fall, but the temperatures can sometimes exceed 40 °C from September to April, with peaks of 48/50 °C in December and January. The climate is milder from May to August, when days are mild, around 20 °C and nights can be cool or even cold (the cold records are around freezing from June to August).
Further north, the hot period is longer, so that highs normally exceed 35 °C from October to March at the Tropic of Capricorn. Some cities are located a few hundred metres above sea level and are therefore slightly cooler, such as Alice Springs, in the area of the MacDonnell Ranges in the Northern Territory, which is located at 500 metres above sea level, and while it's still hot in summer (but with a highest recorded temperature of "only" 45 °C), it can be more easily cold in winter, when at night the temperature can drop a few degrees below freezing, but during the day, when the sun shines, the temperature rises a lot and the air becomes pleasantly mild.
Average temperatures - Alice Springs
Alice SpringsJanFebMarAprMayJunJulAugSepOctNovDec
Min (°C)22211813854611151821
Max (°C)373633292420202328313436

Precipitation amounts to 250 mm per year, mainly due to the rare showers and thunderstorms in the warm period. Here is the average rainfall.
Average precipitation - Alice Springs
Alice SpringsJanFebMarAprMayJunJulAugSepOctNovDecYear
Prec. (mm)45353010151889183040249

The best time to visit this arid area goes from May to August: it can get cold at night, but at least you avoid the extreme heat.

Ayer's Rock

Area with a Mediterranean climate
In these two southern areas the climate is Mediterranean, with mild, rainy winters and warm, sunny summers. The proximity to the ocean makes the summer windy, but at the same time, being not far away from the desert, some heat waves are still possible, with highs around 40 °C.
In Perth, Western Australia, the average temperature goes from 12 °C in July to 24 °C in January and February. In a typical year, 800 mm of rain fall, most of which occur from May to August, with a maximum of 170 mm in July, the central month of winter. In Perth the temperature never drops below freezing, although in the metropolitan area there may be slight frosts in the winter months. From November to March, the hot wind from the desert can raise for a few days the temperature at around 40 °C, with peaks of 45 °C, but normally the temperatures are more pleasant, with highs around 28/30 °C and the afternoon breezes blowing from the ocean.
Average temperatures - Perth
Min (°C)16171512108778101214
Max (°C)313129252118171820222528

The ocean temperature in Perth is cool all year round, but it becomes acceptable from January to May, when it reaches 22/23 °C.
Sea temperature - Perth
Sea (°C)222223232221201919192021

South of Perth, in the south-western tip of Australia, where there are towns such as Denmark and Pemberton, the rains reach 1,000/1,200 mm per year, but still with a Mediterranean pattern (ie with rainy winters and dry summers); here summers are a bit cooler, but short waves of intense heat are possible as well.
In Adelaide, South Australia, the climate is similar to that of Perth, although summer is slightly cooler and winter is less rainy. The average temperature goes from 23 °C in February, to 11.5 °C in July. Here, too, summer is cooled by breezes, and here also from November to March, short heat waves are possible, with peaks around 40 °C. Here are the average temperatures.
Average temperatures - Adelaide
Min (°C)171715131088810111416
Max (°C)293027231916151719222527

The rainfall amounts to 530 millimetres per year, with a maximum in late autumn and winter. Here is the average precipitation.
Average precipitation - Adelaide
Prec. (mm)202025457075656550403025530

The sea temperature in Adelaide is cooler than in Perth, as we can see in the following table. On the other hand, within the Spencer Gulf, the sea a bit warmer.
Sea temperature - Adelaide
Sea (°C)192019181817161515151718

The best times to visit these areas with a Mediterranean climate are spring and autumn, and in particular April-May and September-October, remembering though that May and September are quite rainy.

Area with a tropical climate
The vast northern area has a tropical monsoon climate, with a dry and sunny season ("the dry", usually from May to October), and a rainy and muggy season ("the wet", usually from November to April). The annual rainfall exceeds 400 mm, and is more abundant along the northernmost and the eastern coasts, where they exceed 1,200 mm. The tropical rains occur mainly in the form of downpour or thunderstorm, in the afternoon or evening. In the south-east, where Brisbane is located, winter is cool enough to speak of sub-tropical climate rather than tropical. The vegetation is savanna in the drier areas, while we find rainforests in the wettest part of the north-eastern coast.
In the far north, in Arnhem Land and Cape York Peninsula, the weather is hot, summer-like all year round, while the more you go south, the greater the difference between the seasons.
In Darwin, the capital of the Northern Territory, 1,500 mm of rain per year fall, the majority of which occur from November to early April. The rainiest month is January, with almost 400 mm of rain. On the contrary, from May to September it almost never rains. Here is the average precipitation.
Average precipitation - Darwin
Prec. (mm)385310255951521113501202401487

The heat is intense throughout the year, and it's muggy and sweltering in the rainy season, while it's more bearable in the dry season: from June to August, however, the maximum temperatures are still around 31 °C. Needless to say, this is the best time to visit this area.
Average temperatures - Darwin
Min (°C)252525242220192123252526
Max (°C)323232333231313133333333

Along the north-eastern coast, in a city like Cairns (Queensland), the rains are abundant, so that almost 2,000 mm of rain fall annually, including more than 100 mm from December to April. Here is the average precipitation.
Average precipitation - Cairns
Prec. (mm)3804503902159040303035501101801995

Here, too, summer is hot and humid, while winter is the driest season, but lying more to the south, Cairns in winter it's also more temperate, since the daily average of June and July is 21 °C.
Average temperatures - Cairns
Min (°C)242423222018171819212324
Max (°C)323131292826262728303132

At some distance from the east coast of Queensland, we find the Great Barrier Reef, the world's largest coral reef, even 2,300 km long. In the north-central part of the coast, the sea is warm all year round, as we can see from the water temperatures at Cairns.
Sea temperature - Cairns
Sea (°C)292928282625242425262728

Between Cairns and Innisfail, Mount Bellenden Ker is rain-soaked, so that at its top, at 1,593 metres above sea level, even 8 metres of rain fall per year! Innisfail is the rainiest city in Australia, and it's the only city in the Australian tropical zone not to have a dry season, because even in winter and spring, some showers may occur: on average, 3,500 mm of rain per year fall, with a maximum of even 660 mm in March, and a minimum of 85 mm in September and October.
Proceeding south-east along the coast, the temperature gradually decreases, therefore the climate becomes more enjoyable. In Brisbane, the capital of Queensland, summer from November to March is warm and moist, with some chance of thunderstorms, anyway the amount of sunshine is still good. Winter from June to August is very mild, with average highs around 20 °C, lows around 10 °C, and frequent sunny days; some rain is still possible even in winter, and sometimes it can get cold at night, when the temperature can approach freezing. The average rainfall is 1,100 mm per year, with a summer maximum. Every now and then, there is some risk of heavy rainfall in almost all months of the year, except maybe in August and September. Heat waves with scorching heat from the desert are rare compared with other cities in Australia, but they are still possible, from October to March.
Average temperatures - Brisbane
Min (°C)21201916131191013161819
Max (°C)292928262321202224272829

Near Brisbane, the sea is warm in the summer months: it exceeds 24 °C from December to April, so it's good for swimming, while in winter, from June to September, the sea is cool, around 21 °C, but not impossible.
Sea temperature - Gold Coast
Gold CoastJanFebMarAprMayJunJulAugSepOctNovDec
Sea (°C)262626252422212121222325

The best time to visit this area with a tropical or sub-tropical climate is winter, from June to August or September, because it's less hot and more dry.

Climate of the South-east
In the south-east, occupied by the states of New South Wales and Victoria, the climate is temperate, with rainfall generally not abundant, but well distributed throughout the year. The weather becomes cooler in Tasmania and along the southern coast, but also in the hilly regions of the south-east.
Canberra, the Australian Capital, is located at 600 metres above sea level, and has an average temperature of 20.5 °C in January, and 5.5 °C in July. Summer is therefore not too hot, also because of the altitude, while winter is quite cold, at least by Australian standards (in fact, the winter temperatures resemble those of Madrid, Spain), and sometimes it can be foggy. In winter, frosts and even some light snowfalls are possible, but sometimes a bit of cold at night is possible in summer as well. During summer, even here there can be some heatwaves, in which the temperatures rise to around 40 °C from November to February.
Average temperatures - Canberra
Min (°C)1313117310136911
Max (°C)282724201512111316192226

Rainfall is not abundant, so that it amounts to 630 mm per year, but the rains are fairly well distributed throughout the year, with a relative minimum in winter. Here is the average precipitation.
Average precipitation - Canberra
Prec. (mm)605555505040404555656555630

Sydney, lying on the coast, is much milder, especially in winter: here the average temperature goes from 22 °C in January and February, to 12 °C in July. Rainfall is fairly abundant, since it amounts to 1,200 millimetres per year, and it's more abundant from January to July, although it does not vary much in frequency (in fact, it rains on average for 8/10 days per month); in summer the rains may occur in the form of downpour or thunderstorm. The amount of sunshine is acceptable: it is neither great, nor poor, as befits a mild oceanic climate, which means that even in winter there can be sunny days. The waves of scorching heat are not so rare, and are possible from November to March, often accompanied by strong gusts of dry wind that can cause fires in the hills surrounding the city.
Average temperatures - Sydney
Min (°C)181817141198911131617
Max (°C)262624221916161719222325

At Bondi Beach, near Sydney, the sea is pretty cool in winter, while it becomes quite warm from January to March, when it reaches 24 °C.
Sea temperature - Sydney
Sea (°C)242424232120191919192122


Melbourne is located further south, in the state of Victoria, a bay which then opens into the strait that separates Australia from Tasmania. The ocean makes the climate cool, so the average temperature goes from 20 °C in January to 9.5 °C in July. Even here there is no shortage of heat waves, more intense from November to March, although usually short-lived. Summer is usually pleasantly warm, so that the tennis tournament "Australian Open" is played in January, but the suspension of matches is planned during heat waves. The amount of sunshine is good but not great in summer, while it's just acceptable (considering that it's winter) from May to August, with 4/5 hours of sunshine per day. The rains are not abundant, since they don't reach 600 mm per year, but they are well distributed, with a relative minimum in the summer.
Average temperatures - Melbourne
Min (°C)141413118766891112
Max (°C)262624201714131517192224

The sea temperature in Melbourne is significantly lower than in Sydney, it's also a bit lower than in Adelaide, and remains quite cool even in the summer months.
Sea temperature - Melbourne
Sea (°C)181918171615131313141516

In south-eastern Australia, between Melbourne and Canberra, there is a mountain range, the Australian Alps, culminating in Mount Kosciuszko, Australia's highest mountain, with its 2,228 metres. This area is humid and rainy, cold in winter, even snowy above 1,500 metres, and cool in summer.
There are also resorts which are equipped for winter sports. Here are the average temperatures of Threbdo, located at 1,400 metres above sea level.
Average temperatures - Threbdo
Min (°C)77420-3-4-3-1135
Max (°C)212118141065610131619

Further south is Tasmania, the southernmost island, exposed to the westerly winds, which has an oceanic climate, albeit mild, wetter in the west, where rainfall exceeds 1,200 mm per year. The capital, Hobart is located in the eastern part, and receives only 570 mm of rain, however, well distributed throughout the year, and falling in frequent short showers and drizzles.
Here is the average precipitation in Hobart.
Average precipitation - Hobart
Prec. (mm)453540453545456060555055570

The average temperature goes from 8.5 °C in July, to 17.5 °C in January and February, so winter is mild and summer is cool. Anyway, every now and then during winter a cold air mass can reach the island, and snowfall can occur in inland hills and mountains, while in summer neither Tasmania escapes the hot desert wind, although here this wind comes generally weakened.
Average temperatures - Hobart
Min (°C)13131198556781011
Max (°C)222220181512121415171920

The sea temperature in Tasmania is cold all year round, however, it reaches 17 °C from January to March.
Sea temperature - Hobart
Sea (°C)171717161514131313131415

This area of south-east Australia can be visited in spring and autumn, that is from April to May and September to October, with a preference for the latter, because the autumn (April-May) it's often rainy.

Area affected by cyclones in Australia

Australia can be affected by tropical cyclones, intense low pressure areas that are formed when the sea is very warm and therefore is able to provide the necessary energy. Cyclones bring strong winds and torrential rains, and affect mainly the northern coasts, although sometimes they can penetrate in inland areas, weakening along the way but still bringing heavy rains. The cyclone season runs from November to May, but the phenomenon is more likely from mid-January to late April.

Australia's climate is affected also by the phenomenon known as El Niño, which is associated with more or less severe drought throughout the country, particularly along the north and east coast, where it reduces the summer rains, and in the south, where drought is accompanied by hot winds from the desert which can cause fires. The opposite phenomenon, La Niña, brings rains more abundant than usual in the north and east, especially in spring and summer, and an increase in the number of cyclones.

If you plan to visit the whole country, not just the south but also the vast arid outback and the tropical north, you may prefer winter, from June to August, to avoid the heat, which can hit in other seasons, especially in the north-central, and in summer in the south, but also to avoid rains and hurricanes that can affect the north in summer. Of course, winter in the south is quite cold, it can be very rainy in Perth and in western Tasmania, but if you don't stand or don't like the heat this is the best solution.
If you intend to visit mainly the southern areas, where the major cities are located, and maybe you want to make a quick tour in the central and northern regions without particularly suffering from the heat, you can choose spring and autumn, and in particular April-May and September-October.
The southern areas can be visited in late spring and summer, especially in November-December in Sydney, and from November to February in Melbourne, taking into account that sometimes there may be some scorching days.

What to pack

In winter (June to August). In the tropical north and Darwin, lightweight clothing of natural fibres, sun hat, a scarf for the breeze, a light sweatshirt for the evening and air-conditioned places.
In the desert, spring/autumn clothes, light for the day, sweater, wind jacket, warm jacket for the night, scarf for the sand.
In the north-east (see Cairns, Innisfail), light clothes for the day, a sweatshirt for the evening, possibly a raincoat for the rain showers; for the reef, equipment for snorkeling, water shoes or rubber soled shoes.
In the south-central and the major cities, spring/autumn clothes, sweater, warm jacket for the evening and windy days, raincoat or umbrella. In the southern mountains, warm clothes, fleece, down jacket, gloves. In Canberra and Tasmania, warm clothes, sweater, down jacket, hat.

In summer (December to February). In the tropical north and Darwin, and in the north-east up to Brisbane, lightweight clothing of natural fibres, a scarf for the breeze, a light sweatshirt for the evening and air-conditioned places, a light raincoat for thunderstorms; for the reef, equipment for snorkeling, water shoes or rubber soled shoes.
For the desert, lightweight clothing, loose-fitting and made of natural fabric (cotton or linen), desert turban, sunglasses, a light sweatshirt for the evening, a sweatshirt for the night, sleeping bag for overnight stays outdoors, desert boots or sandals.
In the south-central and the major cities, light clothing, a sweatshirt for the breeze and for cool days, a scarf for the wind-borne sand and dust, possibly a light jacket and an umbrella, especially in Canberra, Sydney and Melbourne. For Tasmania, spring/autumn clothes, sweater, jacket.