Map from Google - Niue

The climate of Niue, a South Pacific island located north of the Tropic of Capricorn, therefore in the Southern Hemisphere, is tropical, with a warmer period, muggy and rainy, from November to April, and a cooler and drier period from May to October, during which the south-east trade winds dominate. During the warmer period, the temperatures hover around 28/29 °C during the day, and around 22/24 °C at night, while in the cool period, daytime temperatures drop to around 25/26 °C, and nighttime temperatures around 20/21 °C. During the warmer period, sometimes the temperature exceeds 35 °C, while in the cool period at night it can drop below 15 °C, especially in July and August.
Here are the average temperatures of the island.
Average temperatures - Alofi
AlofiJanFebMarAprMayJunJulAugSepOctNovDec
AlofiJFMAMJJASOND
Min (°C)232424232221202021212223
Max (°C)282928272626252526262728

The annual precipitation amounts to 2,000 millimetres, with a maximum between January and March, when 250/300 mm of rain per month fall. In the relatively dry period, however, there's no shortage of rains: on average, 80/100 mm per month fall even from June to September. Here is the average precipitation.
Average precipitation - Alofi
AlofiJanFebMarAprMayJunJulAugSepOctNovDecYear
AlofiJFMAMJJASONDY
Prec. (mm)26025030020013080901001001201401901960
Days

Niue can be affected by tropical cyclones from November to mid-May, although they are more likely from December to April.

Niue

The sea temperature is always warm enough for swimming, it only goes down to 25 °C from July to October, as we can see in the following table.
Sea temperature - Niue
NiueJanFebMarAprMayJunJulAugSepOctNovDec
NiueJFMAMJJASOND
Sea (°C)282828282726252525252627

When to go

The best time to travel to Niue is from mid-May to October, being the coolest and sunniest. Sometimes it can be a little cool in the evening and at night, so it may be a good idea to bring a sweatshirt or sweater.

What to pack

In winter (June to August): light clothes for the day, a scarf for the breeze, a sweatshirt for the evening, possibly a light raincoat or umbrella.
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.