Flight cancelled as powerful cyclone bears down on Fiji

"As a nation, we are facing an ordeal of the most grievous kind," Prime Minister Voreqe Bainimarama said in a national address.

"We must stick together as a people and look after each other. Be alert and be prepared."

Severe tropical cylone Winston, with average winds of 220 kilometres (136 miles) per hour and gusts of up to 315 kilometres per hour, was forecast to hit the main island Viti Levu overnight Saturday, according to the Fiji meteorological office.

The storm brushed by the more northerly island of Vanua Levu on Saturday afternoon, with unconfirmed reports of trees felled and roofs blown off houses.

"I've never seen such a strong cyclone as this one in 60 years," local resident Zalim Hussein told AFP by phone.

"This is a monster cyclone, without doubt the strongest we've ever experienced. We're all very scared and hope it goes away quickly."

National Disaster Management director Akapusi Tuifagalele told the Fijilive website that 758 evacuation centres had been prepared in the nation of about 900,000.

Fiji Airways, Virgin Australia and Jetstar all cancelled flights to the holiday hotspot, which is a popular destination for Australians and New Zealanders.

"Our team of meteorologists and safety experts have been closely monitoring Tropical Cyclone Winston and have determined that conditions are not suitable for flying in or out of Nadi, Fiji," Virgin Australia said in a statement.

The government imposed a nation-wide curfew on Saturday night, with Bainimarama saying conditions would be too dangerous for residents to venture outside.

"I ask parents to be especially careful of the young and the elderly," he said.

"Do not allow anyone to go outside during the storm itself. The threat of being hit by flying debris is extremely high."

Winston has been moving around the South Pacific for a week, skirting around Vanuatu and Tonga and has previously taken aim at Fiji before veering away.

Cyclones are common in the South Pacific and their impact is notoriously difficult to predict, with major storms capable of causing major devastation but sometimes blowing themselves out with relatively little damage.

The UN humanitarian agency UNOCHA said it was ready to support the disaster management office if required.