A picture taken and released by the Tonga Police on April 9, 2020 shows damage to a tourist resort caused by Tropical Cyclone Harold in the Hihifo coastal area—police said three resorts were reduced to rubble

A resurgent Tropical Cyclone Harold flattened tourist resorts in Tonga Thursday, extending a week-long trail of destruction across four South Pacific island nations that has claimed more than two dozen lives.

The cyclone gathered pace as it bore down on the tiny island kingdom, which declared a state of emergency, warning residents to seek shelter from destructive winds and massive sea surges.

By early Thursday it had again become a scale-topping Category Five superstorm—surprising meteorologists after signs earlier in the week that its intensity was dropping.

Packing winds of up to 260 kilometres per hour (160 miles per hour), it cut power in parts of the country and police said at least three tourist resorts north of the capital Nuku'alofa had been reduced to rubble.

The cyclone killed 27 people in the Solomons late last week before barrelling southeast to directly hit Vanuatu as a Category Five, obliterating entire towns in the northern provinces.

There have been no reports of deaths in Vanuatu, Fiji or Tonga, with emergency workers saying residents in the hardest hit areas took shelter early.

"It appears that many buildings and crops have been destroyed and some people in the most affected areas have lost everything," Red Cross Vanuatu secretary general Jacqueline de Gaillande said.

Harold weakened slightly to a still-formidable Category Four as it lashed Fiji on Wednesday but hopes the storm was dissipating were dashed when it regathered momentum heading towards Tonga.

"It's been a tricky one to predict," meteorologist Bill Singh from New Zealand's Metservice told AFP.

"We knew the track it was going to take but initially everyone thought it was just going to be Cat 3 or 4, but as it progressed over open warm waters it deepened."

Map showing the path of Tropical Cyclone Harold which sideswiped Tonga on Thursday as a Category Five storm after leaving a trail of destruction across the South Pacific.

Closed borders

The storm was expected to head away from Tonga onto the open ocean late Thursday but WeatherWatch.co.nz head forecaster Philip Duncan said there were no certainties.

"It's almost unheard of to see a cyclone tracking south away from the equator, weakening, then suddenly returning back to Cat 5 so far south," he said.

The COVID-19 pandemic has complicated disaster relief efforts, with Vanuatu reluctant to open its international borders as it seeks to remain one of the few countries without any confirmed virus cases.

"No foreign personnel are being brought to Vanuatu for response efforts at the present time. This will be an internally run operation," Vanuatu's National Disaster Management Office said.

Fiji has 15 virus cases and Prime Minister Frank Bainimarama said the virus and cyclone meant "our economy and our people have been dealt two body blows to start the year".

"This storm must not compromise our coronavirus containment efforts, lest we risk damage far more painful than the aftermath of any cyclone," he said.

Australian Foreign Minister Marise Payne said an air force plane carrying essential supplies such as tents and water containers was on its way to Vanuatu, while assistance had also been offered to other affected nations.

"We are acutely conscious that this comes on top of the impact and difficulties created by COVID-19 for those countries," she said.

Vanuatu said any supplies that came into the country would be handled with protective equipment and the air crews delivering them would be isolated in transit areas.