The cyclone has continued apace through the South Pacific since forming in the Solomon Islands, where it contributed to the deaths of 27 people in a ferry accident.

Thousands of people were made homeless in Vanuatu after Harold made landfall at least twice as a category-five storm with winds of more than 235km/h.

There are now reports of at least one death on Vanuatu's Malo island, and complete devastation on Ambae island.

Cameraman Joseph Tom, who was one of the first to survey the hardest-hit northern islands, told Radio NZ everything was smashed to the ground.

"Fruit trees, coconuts. The rivers, all the mud is in the sea, you can hardly see the reefs. There's lots of damage," Mr Tom said.

The storm weakened to category four as it continued east to Fiji, still producing major destruction.

Harold appears to have made landfall with the southern island of Kadavu on Wednesday afternoon, but as was the case in Vanuatu, communication lines have been disrupted and there will be a wait to assess the situation.

Fiji's main island of Viti Levu was lashed by major rainfall, producing flooding, and winds.

The Australian and New Zealand Governments are supporting the impacted nations with damage assessments and immediate relief.

Foreign Minister Marise Payne said she was conscious "this comes on top of the impact and difficulties created by COVID-19 for those countries".

"We stand ready to provide what further help we can to our Pacific family in whatever ways we can," she said.

New Zealand counterpart Winston Peters said the Kiwi defence forces would take every precaution to ensure they weren't bringing coronavirus to the Pacific.

"They'll be tested of course," he said.

"They've had to suspend their social distancing rules. They can't help each other in a crisis like this without acting much more physically adjacently.

"When you've got outsiders coming in like in our case, we've got to be ultra careful that we've done all we can to make sure that what we're bringing in is coronavirus-free."

New Zealand has pledged $NZ2.5 million of support.

Tonga issued a State of Emergency, but were spared the worst as the re-strengthened cyclone stayed south.

The Associated Press reports a criminal investigation has been opened into the ferry deaths in the Solomon Islands.

Prime Minister Manasseh Sogavere has called it an "unimaginable" tragedy and questioned whether the overcrowded ferry should have been at sea.

Australian Associated Press