SYDNEY/SUVA, FIJI (REUTERS, AFP) - Fiji declared a state of natural disaster on Thursday (Dec 17), ordering its entire population to take shelter ahead of a nightly curfew as a potentially devastating cyclone approached the Pacific Island nation.

Cyclone Yasa, a top category five storm, is expected to bring winds of up to 250 km per hour and torrential rain across the South Pacific archipelago when it makes landfall overnight.

Still a few hours out from the worst weather, Fiji's Meteorological Service said storm force winds and heavy rains have been recorded in some parts of the country already.

Images shared on social media showed roads blocked by landslides, floodwaters and fallen trees.

By 8 pm (4pm Singapore time), the centre of Yasa is forecast to be 100km east of the village of Yasawa-i-Rara and potentially over Fiji's fifth-most populous province of Bua, home to 15,000 people, Fiji's National Disaster Management Office said.

Reinforcing the threat, Fiji Prime Minister Frank Bainimarama told the country's near 1 million population to find safe shelter ahead of a 14-hour nationwide curfew beginning at 4pm (noon SGT).

"The impact for this super storm is more or less the entire country," Bainimarama said in a video posted to Facebook.

Yasa would "easily surpass" the strength of 2016's Cyclone Winston, he added, referring to the southern hemisphere's most intense tropical storm on record, which killed more than 40 Fijians and left tens of thousands of people homeless.

The curfew order was given as part of the state of natural disaster order, which will run for 30 days.

Over 95 per cent of the population live in the direct path of Yasa, said Bainimarama, adding weather forecasts anticipated flash flooding and "severe coastal inundation" that included waves up to 10 metres high.

Residents in the areas deemed most at risk said the warnings were been heeded.

"We'll see what happens," said Alumita Bati, a chef from the capital Suva who was forced to evacuate with her son and husband from their corrugated tin house in a low-lying settlement outside the country's most populous city.

Bati's family boarded up the windows of their house with more tin before going to her sister's house on higher ground where she felt safer but still "a bit scared".

Fiji banned the running of public transport, and was taking precautions with some 50 foreign yachts moored in the southern part of the island chain.

"The boats have been moved to mangrove shelter, which provide good protection against the winds," said Cynthia Rasch, chief executive officer of Port Denarau Marina.

Fiji in October opened to foreign boats in a bid to revive a tourism industry hit hard by the coronavirus. Dubbed the Blue Lane initiative, foreign yachts have to follow stringent requirements to enter Fiji, including a 14-day quarantine out at sea.

Bainimarama, a long-time campaigner for climate action, had no doubt global warming was fuelling such super storms, which were once rare but have become relatively common.

"As the world is getting warmer these storms are getting stronger," he said.

"Every one of us must treat these climate-fuelled catastrophes with deadly seriousness."

Housing in Fiji's villages is largely made from timber and corrugated iron, making them vulnerable to high winds.

Sheltering in schools, church halls and even shipping containers offers better protection but runs the risk of health issues if people stay longer than a few days.

All schools and public transport were closed, with non-essential public servants told to stay home.

"We don't want anybody losing their lives out there," assistant police commissioner Abdul Khan said.

New Zealand-based meteorological service Weatherwatch said Yasa was one of the most powerful cyclones recorded in the South Pacific, capable of cutting a swathe of destruction 300km wide.

"This storm has the ability to swamp entire islands, inundate entire coastal communities, wipe some small islands off the map entirely," Weatherwatch managing director Philip Duncan said.

Save the Children said lessons had been learned from Winston in 2016 and Fijians were taking cyclone preparation seriously.

"We've seen people stocking up on essential food items and supplies," the aid agency's Fiji chief Shairana Ali said.

"People are stocking up on water because there is a warning from the Fiji Water Authority that people will not have proper water supply for at least 10 days."

Ali said Yasa was expected to make landfall late Thursday at Bua province, on the island of Vanua Levu, although the cyclone's track has been difficult to accurately predict.

Fiji's borders remain closed due to the coronavirus pandemic, which Ali said could present a challenge for international relief efforts.

Aid agencies have pre-positioned supplies across the country in anticipation of a major disaster during cyclone season, which runs until May next year.

The most recent category five storm to hit Fiji was the highly destructive Cyclone Harold in April this year.