Map from Google - Brisbane

The climate of Brisbane, capital of Queensland, and lying on the east coast of Australia, is subtropical oceanic, with very mild and relatively dry winters, and warm and rainy summers.
Being in the Southern Hemisphere, like the rest of Australia it has reversed seasons compared with Europe or North America.
Here are the average temperatures.
Average temperatures - Brisbane
BrisbaneJanFebMarAprMayJunJulAugSepOctNovDec
BrisbaneJFMAMJJASOND
Min (°C)21201916131191013161819
Max (°C)292928262321202224272829

Because of its location, in winter the city is more sheltered from the cold, and also from wind and rain, than the southern cities (Perth, Melbourne, Sydney), but it is also protected from desert heatwaves, because in summer the eastern ocean currents are prevailing. But in summer it can sometimes be affected by tropical cyclones, or at least their remains, which in any case are able to bring heavy rains.
In an average year, 1,200 millimetres of rain fall, of which more than 100 mm per month from December to March, while the driest period is winter, along with early spring.
Here is the average precipitation.
Average precipitation - Brisbane
BrisbaneJanFebMarAprMayJunJulAugSepOctNovDecYear
BrisbaneJFMAMJJASONDY
Prec. (mm)16017014090807060403090901201140
Days13151312119778101313131

Winter, from June to August, is very mild: the average daytime temperatures are about 20 °C. At night at certain times it can be cold, so that the temperature can drop to around 3/5 °C, but the days remain mild. There are many fine days, and the rains are quite rare.

Spring, in September and October, is pleasantly warm, but the rains start to increase.

Summer is long, going from November to March, and it's hot and muggy, but tempered by sea breezes, which regularly blow, especially in the afternoon. The weather is often good, but thunderstorms can break out quite often, sometimes accompanied by hail and wind gusts. In the years of La Niña the rains may be particularly abundant, as happened in January 2011, when the Brisbane River flooded the city.
Every now and then, a tropical cyclone can reach the city, although this rarely happens, because cyclones usually pass to the north, beyond the Tropic. Sometimes cyclones can pass at some distance, causing storm surges which can reach the coast.

Autumn, in April and May, is pleasantly warm, and it is quite rainy, although rainfall is generally less intense than the summer thunderstorms. In May, sometimes at night it can get a little cold, with minimum temperatures below 10 °C.

The amount of sunshine in Brisbane is good all year round, since there are several nice days in all seasons.

Brisbane

For swimming, the sea is quite warm from December to April, when the temperature is 24/26 °C, while in winter, from July to September, it's cool, about 21 °C , but in the end it's not impossible. So, if you go in the Southern Hemisphere winter, maybe in July and August, on a nice day you can even try a dive in the sea.
The Goald Coast to the south and the Sunshine Coast to the north of the city are popular with surfers, while the city is located in a bay (the Moreton Bay) which is more sheltered from the waves.
Here are the average sea temperatures.
Sea temperature - Gold Coast
Gold CoastJanFebMarAprMayJunJulAugSepOctNovDec
Gold CoastJFMAMJJASOND
Sea (°C)262626252422212121222325

When to go

Having a good climate, Brisbane can be visited all year round. Being the driest time, you can choose winter and early spring, from June to September. October is a bit more rainy, but is pleasantly warm, without excesses. Summer is hot and muggy, but generally the heat is bearable and you can also swim in the sea. Sometimes, however, there can be severe thunderstorms, and even a cyclone can approach the city.