Three people are seriously injured, buildings have been destroyed and there are "roofs everywhere" in Tonga following Tropical Cyclone Gita's visit last night.

His Majesty's Armed Forces have been clearing debris from roads all morning to allow emergency services to get across the main island of Tongatapu to assess the damage.

The category four cyclone brought winds of more than 233km/h overnight, ripping roofs off houses, destroying buildings and cutting power to the island.

In the capital Nuku'alofa photos have shown Parliament house levelled, and trees snapped in half.

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"Wow, it's a mess," said Graham Kenna of Tonga's National Emergency Management Office.

"There is so much damage, roofs everywhere, it's the worst situation I have been in.

"A lot of the old heritage buildings, some that have stood for over 100 years, have been destroyed.

"A large number of houses have been completely destroyed also."

With the roads cleared, emergency services had branched out to the east, west and the capital to carry out initial assessments of the damage, Kenna said.

"The initial assessment will cover if people have homes to go back to.

"After that we will know just what we are dealing with.

"Later, a more in-depth assessment will cover water, sanitation, then agriculture and what items people have lost."

They had emergency supplies on standby and emergency funds were available.

"Electricity is out on the whole island and will likely take a week to get restored.

"Food is not a problem, as the island produces most of its food, but water will be as many water tanks have been badly damaged and those still standing need electricity to run the pumps."

There should be enough to last the week though, he said.

The aim was to get the hospital fully operational by tomorrow evening.

Three people had been seriously injured by flying debris, and many others had less serious injuries.

There were no confirmed deaths, but "some rumours flying around", Kenna said.

There were also no reports of people trapped in buildings.

"If there were it is likely we would know about it by now."

They had not been in touch with all evac​uation centres but knew at least 1000 families had evacuated ​their homes since yesterday.

"We were in touch with them through the night and they are all safe with plenty of food and water.

"The Tongan people are handling it very well."

A curfew would remain in place in Tonga for the rest of the day.

Tonga Red Cross has also begun assessments, with teams heading out to different parts of the main island Tongatapu and a group of volunteers in 'Eua.

Communications adviser Polikalepo Kefu was part of a group assessing the eastern part of the island.

"We have assessed about five villages so far and there is a lot of damage. People are devastated," Kefu said.

"Some people have lost their homes, and many homes have been badly damaged.

They had not come across any serious injuries yet, but there were reports of minor injuries on the island 'Eua, close to the eastern part of Tonga.

"It was worse than people were expecting. Tonga has experienced nothing like this."