Flash flooding cut highways and forced evacuations in Fiji Friday, with residents sheltering from rising waters on rooftops as authorities scrambled to find rescue boats.

Heavy rains caused rivers to burst their banks in the west of the main island Viti Levu, taking water levels higher than those experienced during a six-day deluge in January which claimed 11 lives, meteorologists said.

Police said they were not aware of any deaths in the latest disaster, which cut off the town of Nadi, home to Fiji's international airport, as well as other centres including Ba, Lautoka, Rakiraki and Sigatoka.

Most flights to and from Nadi were cancelled, national carrier Air Pacific said.

The disaster management office, Dismac, said a "massive" number of people were stranded on rooftops awaiting rescue and appealed for anyone with a boat to help relief efforts.

"We've got a lot of reports of people on rooftops, it's quite a massive number," Dismac director Pajiliai Dobui told reporters.

"If people in these areas have boats, we're asking them to make them available, as the little we have is not enough."

Dismac said it had opened 11 evacuations centres. No figures detailing how many people were sheltering in the centres were immediately available.

The National Weather Forecasting Centre predicted the rain would continue until at least Sunday, accompanied by strong winds on Saturday.