At least three people have been killed and thousands of others made homeless after Cyclone Mick caused flooding and power outages across Fiji, authorities have said.

The cyclone ripped through the most populated island of Viti Levu on Monday night before heading towards Tonga, which has been placed on alert.

Speaking from the capital Suva, Tim Sutton, of the UN Children's Fund (Unicef), said Mick was a "very vicious" cyclone with gale force winds that struck up to six hours earlier than expected.

"There's a lot of damage. Lots of trees down, power lines down everywhere, all the roads around Viti Levu are still closed with flooding and landslides," he told New Zealand's National Radio.

Police said that the dead included a 19-year-old man who was reportedly swept away while crossing a river in northern Viti Levu, a 23-year-old man drowned in the southwest and a student hit by a tree in the northwestern highlands region.

Reports said two fishermen might also be missing near the town of Lautoka on Viti Levu.

Emergency shelters

Fiji's national disaster management office said the death and injury toll had been reduced by people heeding the cyclone warning and flocking to emergency shelters before the storm struck.

Many holiday resorts lay in the path of the region's first cyclone of the southern summer, but foreign visitors were reported to be safe despite some damage to the facilities.

Around 3,000 people sought shelter before the deadly winds and torrential rains hit, but by late Tuesday most had returned to their homes, officials said.

Fiji's meteorological service said winds gusting up to 150km an hour were recorded close to the Category Two cyclone's epicentre.

Category One is the lowest cyclone rating while five is the highest.

The disaster management office had still not heard from some small outer islands after contact was lost during the storm, officials said.

Fiji is an archipelago of more than 320 islands.