Cyclone described as the strongest to hit Tonga in its history.
Some estimates says 75 per cent of homes in capital Nukuʻalofa destroyed.
Fresh water and power are out in many areas.
New Zealand Defence Force planes and emergency response teams are on standby
If you're a Kiwi in Tonga and require consular assistance contact the NZ High Commission on (00676) 23122, (00676) 881 7022 (after hours) or nzhctonga@mfat.govt.nz
The extent of the destruction in Tonga caused by Cyclone Gita is starting to emerge.
The storm tore through the country overnight, injuring people and destroying buildings. Tonga's parliament house is one of the buildings destroyed.
A spokesman for Tonga's Emergency Management office told Newshub they have reports of one death, but it cannot yet be confirmed because communications are down.
Graham Kennatold Newstalk ZB's Mike Hosking there was an extreme amount of damage in capital Nuku'alofa's CBD.
He said a lot of heritage buildings in the town had been severely damaged or destroyed.
Kenna said about 800 families were sheltering in evacuation centre. All were safe.
He described the storm as "frightening". He was on the third floor of one of the strongest buildings in Nuku'alofa all night and it was shaking and being continually battered by flying debris.
His Majesty's Armed forces started clearing the roads around the hospital from about 2am.
Damage in Tonga after Cyclone Gita hit the island nation overnight. Photo / Tonga Now Facebook
A number of people are injured and in a woman in labour had to be rushed to hospital.
Kenna said it was still dangerous in the area. Strong winds were blowing tin around from buildings.
Kenna expected power would be out for a number of days and that many people's water tanks would have been damaged.
He told Radio New Zealand that one of the Catholic churches was completely gone and others were damaged.
"We've had hundreds of calls during the night for assistance for trees down on houses and people trapped in houses," he said.
"I've had phone calls through the night from people who were trapped and wanted help that we just weren't able to offer. We couldn't put lives at stake to go out in such atrocious weather."
Destruction after Cyclone Gita hit Tonga. Phto / Tonga Now Facebook
Kenna told RNZ that staff from the emergency management office would head out to help people and assess the damage as soon as it was safe.
"We'll start in the city and then we'll fan out into the countryside and get a full grasp by mid-afternoon on what the needs are going to be."
New Zealander Joanna Bourke, who lives in Tonga, said she was "fearing the worst".
She told Newstalk ZB Early Edition host Kate Hawkesby that the night was "horrible" as the cyclone "roared".
Tonga had been badly hit, she said, although she would have to assess damage to her own home after dawn.
She had looked outside with the use of a torch and "it didn't look good" with of wood everywhere, coconut fronds and fallen trees.
Bourke described herself as a tough cookie, but conceded that when the cyclone was roaring through, she was scared.
Polynesia country director for Oxfam, Jane Foster, told Newstalk ZB that providing access to clean water will be a priority.
Many residents rely on water tanks, which have been damaged or destroyed in the category four storm overnight. Oxfam has water filtration systems ready for use.
New Zealand's 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 today.
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.
