More than 3,500 people evacuated to emergency shelters in Fiji as the biggest cyclone in 20 years swept across the Pacific island nation on Monday, three days after the storm killed four people and destroyed thousands of homes in nearby Samoa.

Tourist resorts on many of Fiji's palm-fringed islands have been evacuated and authorities warned people to remain in shelter as Tropical Cyclone Evan battered the country, blowing over trees and destroying houses.

Authorities said Cyclone Evan had generated destructive winds, torrential rains and was likely to lead to flooding due to a storm surge as it passes to the northwestern side of the main Fiji islands of Vanua Levu and Viti Levu, with wind gusts up to 270 kmh (170 mph).

Fiji's weather bureau said cyclone Evan was rated a category four storm, the second highest level, and was moving only at about 18 kph (11 mph), meaning the destructive winds could last several hours.

Prime Minister Frank Bainimarama ordered public servants to stay at home and he put emergency services on standby. Hospitals and health centres have been closed for all but emergency patients.

Power supplies have also been cut to some areas as a precaution against falling power lines, including in the main tourist town of Nadi. Airlines grounded flights to and from Fiji, stranding about 1,900 visitors in the country.

"I cannot stress enough how serious this is. Every Fijian will be affected but we must take preventative steps now," Bainimarama said.

Residents and businesses stocked up on food and put up shutters to protect shops and offices. Major roads have been closed and authorities are warning that bridges could be swamped by flood waters.

Schools throughout the country were also being used as evacuation centres, with authorities saying more than 3,500 people had sought shelter by late on Monday.

The Fiji Times reported rough seas and ferocious winds had forced a bulk carrier to run aground on a reef near the capital of Suva.

Australia and New Zealand offered support to Fiji ahead of the storm and have search and rescue personnel on standby. – Reuters