Fiji is braced for the arrival of Cyclone Winston, in what could be the Pacific island nation's most powerful storm on record.

The cyclone could hit the main island of Viti Levu overnight local time on Saturday, meteorologists said.

Flights have been cancelled, evacuation centres activated and a nationwide curfew put in place.

Prime Minister Frank Bainimarama warned that his nation was facing "an ordeal of the most grievous kind".

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

Cyclone Winston has already brushed some of Fiji's smaller islands, causing minor damage.

It is hoped the storm could pass through main islands Vanua Levu to the north and Vitu Levu to the south, avoiding a direct hit.

About 900,000 people live on Fiji.