There are fears entire villages have been destroyed after Tropical Cyclone Winston struck Fiji's main island, bringing damaging winds and huge ocean swells to the capital Suva.

Key points: Category five Cyclone Winston bringing winds of up to 325kph

Category five Cyclone Winston bringing winds of up to 325kph Main island hit, storm tracking west-south-west

Main island hit, storm tracking west-south-west Nation-wide curfew imposed and state of national disaster declared

Nation-wide curfew imposed and state of national disaster declared Fears for those outside of major city centres

The Fiji Meteorological Service said the category five storm hit the north-east coast of the main island Viti Levu and tracked westwards.

There were reports entire villages along the eastern coast were destroyed and fears for the safety of those living in remote areas.

"Most of the population around the northern coast of Viti Levu will experience destructive winds," chief meteorologist Ravind Kumar said.

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He said the rest of the country would also experience "very destructive" winds and "torrential rainfall" as the cyclone continued to move.

Jone Tuiipelehaki, a communications officer for the UN Development Program, tweeted: "Some villages have reported that all homes have been destroyed."

"50 homes have been reported destroyed in Navaga village in Koro Island," he added.

At 11:00pm Saturday (local time) the storm was about 50 kilometres north-north-west of Nadi, the country's main transportation hub on Viti Levu's west coast.

The Fijian Government issued a curfew for the whole of Fiji, taking effect from 6:00pm Saturday local time.

"This latest measure is designed to restrict movement and ensure the safety of all Fijians. Only essentials services will be permitted to travel," it said in a statement.

As night fell, several areas of the country were without power, with the Government acknowledging water, power and communications outages would not be repaired until the cyclone passed.

Gloomy skies above Suva, Fiji as residents prepare for the worst of Cyclone Winston. ( Joe Yaya )

The storm, thought to be one of the strongest to ever hit the Southern Hemisphere, has already lashed the outer islands of Fiji after hitting Tonga twice earlier in the week.

The Fiji Meteorological Service issued a warning for the second-biggest island Vanau Levu and the northern half of the main island Viti Levu.

Fijian Cabinet declared a state of national disaster for the whole nation, effective for 30 days, meaning all government forces would be mobilised to deal with the event.

At its centre, Cyclone Winston is generating gusts of up to 325kph, with average winds of 230kph.

Nadi is home to the country's international airport and many airlines have been forced to cancel flights in and out of the country.

Initial reports from the outer islands suggested significant damage to infrastructure, although formal assessments had not confirmed the extent of the damage.

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Fears for those outside of urban centres

Meteorologist Neville Koop from the Na Draki Weather Service said many people outside the main urban centres lived in simple structures so the damage was likely to be significant.

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"A lot of the less well-constructed buildings are going to fail," he said.

"If they fail the people inside will be exposed and try and seek shelter elsewhere and when that starts to happen that's when you run the risk of damage, injury and possibly loss of life."

The town of Rakiraki, halfway between Suva and Nadi, prepared for the worst.

"People are prepared but they are worried the cyclone might bring a lot of damage. The wind is getting quite strong now," Rakiraki resident Mohinesh Prasad said earlier.

More than 700 evacuation centres have been set up across the country.

Alice Clements from UNICEF Pacific in Suva said there was a risk the shelters may not be strong enough to withstand the winds.

"The Government's very well prepared but the risk is that a storm of this strength … nature is truly a force to be reckoned with when it's this strong," she said.

The town of Nausori, near Suva, was one of several across the country to experience a power outage.

"It's completely dark outside ... our house is concrete but we can feel the house shaking," resident Kris Prasad said, adding he had seen trees uprooted and flying across neighbours' yards.

"Some people may not be prepared, especially for us in the central division, we haven't had a cyclone make an impact directly on us for a few years."

A hurricane warning was in place for Taveuni and nearby smaller islands, Ovalau, Gau, Batiki, Nairai, Koro, Vanuabalavu, Yacata, Mago,Cicia, Tuvuca, Nayau and Vanuavatu.

A storm warning remains in place for Lakeba, Oneata, Moce, Komo,Namuka, Ogea, Moala, the southern half of Viti Levu, Yasawa and Mamanuca group.

Flood warnings were issued throughout the chain of islands, including for sea flooding for low-lying areas.

A gale warning remains in force for the rest of Fiji.

The United Nations Office for the Cooperation of Humanitarian Affairs said it was ready to respond to requests for assistance.