Tropical Cyclone Harold's trail of destruction through four Pacific nations could threaten a rise in disease, authorities warned Friday, as stretched health services struggled under the added burden of the coronavirus.

Speedy repairs and recovery were needed to prevent malaria and dengue outbreaks following the cyclone which killed 29 and left thousands homeless, officials said.

Over a seven-day period, Harold developed from a category one cyclone into a category five super storm as it slammed into the Solomon Islands, Vanuatu, Fiji and Tonga.

Homes and crops were destroyed, plantations felled, and roads were covered in debris including several wrecked boats.

Sami Mafi told AFP the storm had forced waves directly into his home, near Fanga'uta Lagoon in Tonga, as his family hauled sodden mattresses, clothes and belongings from inside.

"It wasn't the rain, it was the sea came in. The water hit the house, the door opened, my family were swimming inside," Mafi said.

Tropical Cyclone Harold  flattened tourist resorts in Tonga and left a trail of destruction across Pacific island nations
Tropical Cyclone Harold flattened tourist resorts in Tonga and left a trail of destruction across Pacific island nations TONGA POLICE / Handout

With the arrival of Harold coinciding with a king tide in Tonga, Mafi's fishing boat was swept 200 metres (yards) inland where it was left overturned and half buried in a pile of debris.

The Red Cross communications manager for the Pacific, Carl Gustav Lorentzen, described the damage in the region as "substantial" but said it could be some days before a dollar figure could be put on it.

Many areas would be without power for at least a week and in some of Vanuatu's more remote islands it could be two weeks before electricity is restored, Lorentzen said.

"Crops have been 100 percent damaged, houses destroyed and we are having quite some challenges providing shelter in those areas.".

Map showing how Tropical Cyclone Harold cut across the South Pacific this week
Map showing how Tropical Cyclone Harold cut across the South Pacific this week AFP / STAFF

In Luganville, Vanuatu's second-largest town, two people had been killed in the storm which destroyed 70 percent of buildings and left a dire need for clean water and shelter, local MP Matai Seremaiah said.

"If we don't clear up the yards they start to attract mosquitoes and then the next thing, we have malaria or dengue fever coming in," Seremaiah told Radio New Zealand.

In the Solomon Islands 27 people died after being swept off an inter-island ferry.

In Fiji, three days after the storm pounded its way through, 4,000 people remain in evacuation centres.

Several coastal tourist resorts were wiped out along with fishing boats in Tonga.

"The sea was running inland about one metre above its usual highest level, which was quite devastating," Tonga's director of meteorology Ofa Fa'anunu said.

The global 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 cases of the virus.

Fiji has 15 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 Thursday.

Australia and New Zealand have immediately responded to pleas for aid with relief supplies including blankets, tests, hygiene kits and lanterns.

"We stand ready to provide further help to our Pacific family in whatever ways we can," Australian Foreign Affairs Minister Marise Payne said.

"It will be some time before the full impact of this disaster is known."