Map from Google - Tonga

In Tonga, an 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, even May and November are hot and rainy, and even in the coolest 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, brief showers and thunderstorms may occur throughout the country, which fortunately do not reduce too much the sunshine hours. Generally, the rainiest month is March.

Division into groups of the Tonga Islands

In the northernmost islands (1 on the map), which lie at a latitude of about 15 degrees south (Niuafo'ou, Niuatoputapu and Tafahi), the climate is hot and humid all year round, so that we can speak of equatorial climate: between the hottest months (February and March) and the least hot months (July and August), the difference is little: highs from 31 °C to 29 °C (88 °F to 84 °F).
Here are the average temperatures of Niuafo'ou.
Average temperatures - Niuafoou
Min (°C)242424242423232223232324
Max (°C)303131303029292929293030
Min (°F)757575757573737273737375
Max (°F)868888868684848484848686

Annual precipitation is abundant and exceeds 2,300 millimetres (90 inches), with a rainier period from November to April, when it exceeds 200 mm (8 in) per month, two transitional months, May and October, with 170/180 mm (6.7/7.1 in) per month, and a less rainy period from June to September, with 100/140 mm (4/5.5 in) 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 (2 on the map), the annual rainfall decreases to 2,100 mm (83 in), and the temperature in the cooler months drops a bit as well: highs are around 27 °C (81 °F) in July, August and September, though 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 still around 100/120 mm (4/4.7 in) per month. Here are the average temperatures in Vava'u.
Average temperatures - Vavau
Min (°C)232424232221202021222223
Max (°C)303131302828272727282930
Min (°F)737575737270686870727273
Max (°F)868888868282818181828486

For swimming, the sea is warm all year round, reaching a minimum of 25 °C (77 °F) in August and September.
Sea temperature - Vavau
Sea (°C)282828282726262525262728
Sea (°F)828282828179797777798182

More to the south we find the Ha'apai group (3 on the map), made up of islands such as Lifuka, Foa, Ha'ano. Here the rainfall drops to around 1,600 mm (63 in) 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

Maximum temperatures go from 30 °C (86 °F) in February and March, to 26 °C (79 °F) 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
Min (°F)757575737270686868707373
Max (°F)868686848179797979818284

In the far south, we finally find the group of Tongatapu (4 on the map), where the capital Nuku'alofa is located, and the group of 'Eua. Here precipitation is still around 1,600/1,700 mm (63/67 in) per year, but for the more southern position (they are a latitude of 21 degrees south), winter begins to be a cool season: highs from June to September are around to 24/25 °C (75/77 °F), and sometimes they can be affected by cold air masses, able to drop the temperature by a few degrees.
Average temperatures - Nukualofa
Min (°C)232424222020181819202123
Max (°C)293030292726252525262829
Min (°F)737575726868646466687073
Max (°F)848686848179777777798284

In the southernmost islands, the sea temperature also drops to 23 °C (73 °F) in August and September, therefore it remains acceptable but not so warm. Given that the minimum temperature remains generally high, around 18/20 °C (64/68 °F), the conditions remain usually acceptable, but for those who are sensitive to cold, it could feel a bit cool. So in these southern islands of Tonga, you can choose the months of May, October and November, when daytime temperatures are a little higher, at around 26/28 °C (79/82 °F).
Sea temperature - Nukualofa
Sea (°C)272727272625242323242526
Sea (°F)818181817977757373757779


The islands of Tonga are in the path of tropical cyclones. Cyclones are usually formed from November to mid-May, though they are most likely from late December to early April.
Some intense cyclones that hit the archipelago have been: Ron in January 1998, Waka in December 2001, Heta in January 2004, and Gita in February 2018.

When to go

The best time to travel to Tonga goes from June to September in the northern islands, down 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 also in November and the first half of May, for safety you can exclude these periods, even where they fall within 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.