The cyclone roared through the South Pacific after 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 at least twice on Monday made landfall as a category-five storm, with winds surpassing 235km/h.

The storm weakened to category four as it continued east to Fiji and by Friday morning was moving at a slower speed of about 110km/h.

Harold appears to have made landfall with the southern Fijian island of Kadavu on Wednesday afternoon, with communication lines disrupted.

Australian Foreign Minister Marise Payne said the country stood with its Pacific neighbours affected by Harold and would help wherever possible.

The federal government would provide supplies to Vanuatu such as blankets, lanterns and shelter and hygiene kits, and would also support the Vanuatuan government's health, education and police sectors.

The supplies would be flown into Vanuatu by the Australian air force and Ms Payne said extra care would be taken to avoid COVID-19 transmission.

"We stand ready to provide further help to our Pacific family in whatever ways we can," Ms Payne said in a statement on Friday.

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

New Zealand has pledged $NZ2.5 million of support and Ms Payne's Kiwi counterpart Winston Peters said his country's defence forces would seek to ensure they weren't transmitting coronavirus to the Pacific.

"They'll be tested of course," Mr Peters 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."

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

The Associated Press reports a criminal investigation has been opened into the 27 ferry deaths in the Solomon Islands, with Prime Minister Manasseh Sogavere labelling the incident an "unimaginable" tragedy.

Australian Associated Press