Authorities in Fiji have received reports of "a few" deaths in the devastating cyclone, National Disaster Management Office director Pajiliai Dobui says.
"I think some lost their lives but it is just a few but what we have been hearing from some of the islands is the devastation and the wind and the storm surges were too much," Dobui told Agence France-Presse on Tuesday.
He said the numbers could not be confirmed by police until communications were restored with affected islands in the Lau group and the island of Taveuni, to the east of the main islands of Viti Levu and Vanua Levu.
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Early on Tuesday, Tomas was wreaking havoc some 200 kilometres east of Suva, said senior forecaster for the Fiji Meteorological Service, Matt Boterhoven.
"It's at its closest point to the nation's capital," he told AAP.
"Most of the action" was in the Lau group of islands, he said, with wind gusts peaking at about 230 kilometres per hour.
Relief co-ordinator for the Fiji Disaster Management Office, Anthony Blake, told AAP 17,000 people had sought shelter in over 240 evacuation centres set up around the South Pacific island nation.
On Monday evening the category four cyclone battered Fiji's northern division - destroying over 50 homes and causing "significant other damage".
"Power, water and sewer is out in most places," Mr Blake said from his office in Suva, where he described the weather as windy and overcast.
Communication lines were also down, making it difficult to evaluate the full extent of the damage.
"We should have a better picture in the next 24 hours," he said.
The nation's Disaster Council, chaired by Fiji's self-appointed prime minister Frank Bainimarama, declared a state of natural disaster for the northern and eastern parts of the country on Tuesday morning, Mr Blake said.
Cyclone Tomas claimed one life on Sunday when a woman drowned as huge waves crashed into the Vanua Levu coast.
Mr Boterhoven said he'd heard reports of eight metres storm surges and rising rivers in northern and eastern parts of the country.
Australian Minister for Foreign Affairs Stephen Smith told Sky News people planning to travel to Fiji should "proceed with caution".
"We are, as we always are with Fiji, ready, willing and able to provide assistance if that is required as we have in the past with floods and cyclones and the like," he said.
The cyclone was heading south and by about 10pm (1900 AEDT) would be 300 kilometres south-west of Suva - avoiding any impact on the popular tourist destination of Nadi, Mr Boterhoven said.
It should leave Fijian waters by Wednesday morning.
