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 82/84 °F during the day, and around 72/75 °F at night, while in the cool period, daytime temperatures drop to around 77/79 °F, and nighttime temperatures around 68/70 °F. During the warmer period, sometimes the temperature exceeds 95 °F, while in the cool period at night it can drop below 59 °F, especially in July and August.
Here are the average temperatures of the island.
Average temperatures - Alofi
AlofiJanFebMarAprMayJunJulAugSepOctNovDec
AlofiJFMAMJJASOND
Min (°F)737575737270686870707273
Max (°F)828482817979777779798182

The annual precipitation amounts to 80 inches, with a maximum between January and March, when 10/12 in of rain per month fall. In the relatively dry period, however, there's no shortage of rains: on average, 3/4 inches per month fall even from June to September. Here is the average precipitation.
Average precipitation - Alofi
AlofiJanFebMarAprMayJunJulAugSepOctNovDecYear
AlofiJFMAMJJASONDY
Prec. (in)10.29.811.87.95.13.13.53.93.94.75.57.577.2
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 77 °F from July to October, as we can see in the following table.
Sea temperature - Niue
NiueJanFebMarAprMayJunJulAugSepOctNovDec
NiueJFMAMJJASOND
Sea (°F)828282828179777777777981

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.