the toll was expected to rise as a search was launched for eight men still missing on three fishing boats.
Only one survivor has been found, the New Zealand Rescue Co-ordination Centre, which is overseeing the search, said.
After crossing Samoa, Evan intensified as it ploughed through the Pacific and forecasters said destructive winds could reach nearly 300 kilometres per hour (186 miles per hour) by the time it hits Fiji early Monday morning.
Government officials fear it could be as devastating as Cyclone Kina which killed 23 people and left thousands homeless in 1993.
Squally thunderstorms were expected to flood low-lying areas while coastal villages were at risk of sea flooding, authorities said.
Tourists in luxury resorts on outlying islands were being ferried to the mainland, while Fiji's main airline, Air Pacific, said it had either cancelled or rescheduled its Monday flights.
Philip Duncan, head analyst with the WeatherWatch.co.nz meteorological service, said Fiji could expect to be walloped by the storm with the prospect of flash flooding and mudslides.
"Gusts may end up climbing to 280 kilometres per hour or greater around the centre of Evan," Duncan said.
"Some small, low-lying communities and resorts may suffer catastrophic damage and some small islands may be entirely submerged as the storm and storm surge roll by."
More than 200 evacuation centres have been opened and Information Ministry permanent secretary Sharon Smith-Johns said people at risk should move.
"People living in low-lying areas should consider moving to higher grounds or evacuations centres," she said.
"By sunset tonight everyone should be ready with torches, batteries, candles, supplies and other necessities."
Fiji's military leader Voreqe Bainimarama has warned the storm is an "impending disaster" and offers of international aid have already been received.
Australian foreign minister Bob Carr said Canberra was offering financial aid as well as expert personnel and supplies.
"We're going to work with other nations including New Zealand and France, in doing what we can to save lives, and support search and rescue," he said.
Meanwhile, it could be some days before the full extent of the cyclone damage in Samoa is known because of the difficulty reaching outlying islands.
About 4,500 people have been forced to remain in emergency shelters after Evan destroyed houses and damaged electricity and fresh water supplies, Samoan officials said.
Prime Minister Tuilaepa Sailele also warned of possible food shortages next year because of the destruction of crops.
The New Zealand Air Force and Tahiti search and rescue authorities were scouring the ocean for the fishing boats missing in rough seas.
New Zealand search coordinator Tracy Brickles said the 30-year-old skipper of one of the boats was known to have survived and made his way ashore after his boat tipped over on Friday but there was no information about his crew.
A vessel fitting the description of another boat has been seen washed up on an island but there was no sign of survivors.