New Zealand is poised to rush supplies to Tonga after the Pacific kingdom was hit by Cyclone Gita.

The category 4 cyclone battered the country on Monday night, and a state of emergency was declared amid reports of widespread damage.

A Ministry of Foreign Affairs and Trade emergency management team and Defence Force aircraft were at the ready, Prime Minister Jacinda Ardern told TV3's The AM Show on Tuesday.

Officials were waiting for first light so the damage can be surveyed.


"We will be waiting for the Tongan government to tell us exactly what their needs are, as soon as they are able, and then we can essentially kick in our response as soon as possible," Ardern said.

Radio NZ International was ready to help if communications went down, she said.

Gita has already been through Samoa but New Zealand hadn't been asked for help, Ardern said on Monday.

The Metservice said it expects Cyclone Gita to peak at a Category 4 event, but other regional meteorological agencies said the storm would reach Category 5, the most destructive wind-speed ranking.

Aid agencies warned of potential damage from heavy rain and winds, which the Metservice said could reach up to to 185km/h.

Schools and workplaces in Tonga, which has a population of over just over 100,000, were closed ahead of the storm.

The Red Cross said residents were racing to identify strong buildings to be used for evacuations and to clean up debris such as loose wood or metal that could turn into dangerous missiles in powerful winds.

The Metservice has also warned of huge coastal swells and flooding of low-lying areas.