Map from Google - Tonga

In Tonga, Oceanian archipelago that lies south of the equator, the climate is tropical, influenced by the trade winds, with a hot, humid and rainy season from December to April, and a cool and relatively dry season from June to October. In the northernmost islands, also May and November are hot and rainy, and even in the cooler months the temperature is high; on the contrary, in the southern islands, May and November fall within the relatively dry season, while the period from June to September is pretty cool. However, even in the relatively dry season, throughout the country brief showers and thunderstorms may occur, which fortunately do not reduce too much the sunshine hours. Generally the rainiest month is March.

In the northernmost islands, which lie at a latitude of about 15° (Niuafo'ou, Niuatoputapu and Tafahi), the climate is hot and humid all year round, so that we can speak of equatorial climate: between the warmest months (February and March) and the least hot months (July and August), the difference is little: the maximum goes from 31 °C to 29 °C.
Here are the average temperatures of Niuafo'ou.
Average temperatures - Niuafoou
Min (°C)242424242423232223232324
Max (°C)303131303029292929293030

The annual precipitation is abundant and exceeds 2,300 millimetres, with a rainier period from November to April, when it exceeds 200 mm per month, two transitional months, May and October, with 170/180 mm per month, and a less rainy period from June to September, with 100/140 mm per month. In this area, therefore, the best period goes from June to September, being the least rainy, and albeit slightly, the least hot.
Here is the average precipitation.
Average precipitation - Niuafoou
Prec. (mm)3052503052501651101351151451752302702455

Further south, in Vava'u and the surrounding islets, the annual rainfall decreases to 2,100 mm, and even the temperature in the cooler months comes down a bit: the maximum drops to around 27 °C in July, August and September, but it's still warm enough for a beach holiday, so here too the best period is from June to September, when however the rains are around 100/120 mm per month. Here are the average temperatures in Vava'u.
Average temperatures - Vavau
Min (°C)232424232221202021222223
Max (°C)303131302828272727282930

The sea is warm all year round for swimming, reaching a minimum of 25 °C in August and September.
Sea temperature - Vavau
Sea (°C)282828282726262525262728

Further south we find the Ha'apai group, comprised of islands as Lifuka, Foa, Ha'ano. Here the rainfall drops to around 1,600 mm per year, at least in the flat islands, because there are also volcanic islands, such as Kao, with inland slopes that enhance the formation of thunderstorm clouds.
Here is the average precipitation in Lifuka.
Average precipitation - Lifuka
Prec. (mm)1651952301901058090100110901151501620

The maximum temperatures range from 30 °C in February and March, to 26 °C from June to September. Being the relatively dry season a little longer, here the best time is from May to October. Here are the average temperatures.
Average temperatures - Lifuka
Min (°C)242424232221202020212323
Max (°C)303030292726262626272829

In the far south, we finally find the group of Tongatapu, where the capital Nuku'alofa is located, and the group of 'Eua. Here precipitation is still around 1,600/1,700 mm per year, but for the more southerly position (they are a latitude of 21 degrees south), winter begins to be a cool season: the maxima from June to September are around to 24/25 °C, and sometimes cold air masses, able to drop the temperature by a few degrees, can arrive.
Average temperatures - Nukualofa
Min (°C)232424222020181819202123
Max (°C)293030292726252525262829

In the southernmost islands even the sea temperature drops to 23 °C in August and September, therefore it remains acceptable but not so warm. Given that the minimum temperatures remain generally high, at around 18/20 °C, the conditions remain usually acceptable, but for those who are sensitive to cold, it could be a little cool. So in these southern islands of Tonga, you can choose the months of May, October and November, when the daytime temperatures are a little higher, at around 26/28 °C.
Sea temperature - Nukualofa
Sea (°C)272727272625242323242526

The islands of Tonga are in the path of tropical cyclones, whose season runs form from November to mid-May, although they usually pass from December to April. A particularly intense cyclone that affected Tonga, was Ron, which hit the northern islands in January 1998.


When to go

The best time to travel to Tonga goes from June to September in the northern islands, up to the group of Vava'u inclusive, from May to October in the Ha'apai group, and finally corresponds to the months of May, October and November in the southern group of Tongatapu.
Anyway, considering that, albeit rarely, cyclones may occur even in November and the first half of May, to be sure you can exclude these periods from the planning of the trip, even where they are part of the best period.

What to pack

For the northernmost islands, all year round, tropics-friendly, loose fitting clothing, sun hat, a scarf for the breeze, a light sweatshirt for the evening, light raincoat or umbrella.
For the central and southern islands:
In winter (June to August): light clothes for the day, a scarf for the breeze, a sweatshirt for the evening, possibly a light jacket for the southern islands; light raincoat or umbrella. For the reef, equipment for snorkeling, water shoes or rubber soled shoes.
In summer (December to February): tropics-friendly, loose fitting clothing, sun hat, a scarf for the breeze, a light sweatshirt for the evening, light raincoat or umbrella.