Cyclone Rene battered the main island of Tonga on Monday after earlier sweeping over islands in the north, causing extensive damage.
The Category 3 cyclone damaged houses, knocked out power and flattened banana plantations in the Vava'u group, before hitting the main island of Tongatapu on Monday night.
Wind gusts of up to 230km/h had been forecast for the southern part of the Ha'apai group and Tongatapu.
Maliu Takai, of the National Disaster Committee, said the storm hit about 7.20pm and it was too dangerous to go outside.
Police Commander Chris Kelley says the capital Nuku'alofa was battered by very strong winds and torrential rain for about 50 minutes. Trees have fallen across power lines and buildings have been damaged.
Shortly after 8pm, Mr Kelley said the storm seemed to be all over and it was eerily calm in the capital.
However, Radio New Zealand International correspondent Mateni Tapueluelu says it appears they were just in the eye of the storm and after a short lull the wind began blowing in the other direction.
Phone lines have gone dead late on Monday night. Evacuation plans are in place and people have boarded up their homes.
A businessman on an island in the Vava'u group says the cyclone has left yachts on rocks, houses without roofs and thousands of banana trees flattened. Henk Gros says there will be no power for days as all the lines are down.
The Fiji Met Service says Cyclone Rene is not expected to move away from Tongatapu until Tuesday morning.
NZ prepared to help
New Zealand Prime Minister John Key says the Government is monitoring the situation in Tonga.
Mr Key says talks were held with the Tongan government, Australia and France on Monday about what response might be needed.
About 300 New Zealanders are believed to be in Tonga.
A New Zealand Air Force Hercules has left on Monday afternoon for the Cook Islands following Cyclone Pat last week.
It is carrying relief supplies including tarpaulins and tents, as well as Defence Force personnel who will help with the clean-up on the island of Aitutaki.