Disaster management officials in Fiji say they ha've received reports of several deaths in the powerful Category 4 cyclone that has battered the island nation for four days.
At least 50 homes have been destroyed and 17,000 people have been evacuated to 240 shelters.
Fiji's interim government has declared a state of disaster and New Zealand has announced it is sending an airforce Hercules to assist in relief efforts.
Cyclone Tomas is being described as the worst in living memory in the archipelago.
Late on Tuesday night, the Fiji Meteorological Service was tracking the cyclone as it travelled southwards over the southern part of the Lau group at about 20km/h.
A cyclone forecaster, Alipate Waqaicelua, says it had downgraded the cyclone to a Category 3 storm on Tuesday evening, but it was still packing wind gusts of up to 200km/h.
Mr Waqaicelua says it had just one more island to go past in the next few hours before going into the open sea. The Lau group is to the east of the main islands of Viti Levu and Vanua Levu.
Strong swells were continuing in the Central Division, Lomaiviti and Lau.
Mr Waqaicelua says although conditions will be much calmer on Wednesday, winds will still be blowing strongly as aid efforts begin.
Awaiting verification
The National Disaster Management Office in Suva says it will be able to verify reports of several deaths when communications are restored on Wednesday.
It has so far confirmed 50 homes - as well as hospitals and other public buildings - have been destroyed, but the final number is expected to be at least three times that.
Operations manager Anthony Blake says airport, police, and health facilities have all been damaged.
Director Pajiliai Dobui says police cannot confirm numbers until communications are restored with affected islands in the Lau group.
A state of natural disaster has been declared in the Northern and Eastern divisions. The interim government has also extended a nationwide curfew until Wednesday morning.
Mr Blake says the declaration of a state of disaster frees up resources for the affected areas, and police, army, and other emergency services are being deployed.
He says there has been widespread flooding and some roads are closed. Power is also out in most parts of the Northern Division, which he says is affecting water and sewerage systems.
Winds up to 270km/h
The storm lashed northern parts of the Pacific nation on Monday, producing winds of up to 270km/h. It is said to have generated waves up to seven metres high off Vanua Levu, damaging the island and cutting power.
The Meteorological Service says the cyclone was so fierce it broke a strong sea wall and flooded villages on outlying islands to the east of Viti Levu.
Meteorologist Matt Boterhoven says people on the island of Lakeba were evacuated to higher ground overnight on Monday and a sea wall on the island of Ovalau, which had never been broken before, was destroyed.
Mr Boterhoven says the full extent of the damage there will not be known until people can inspect their properties, and communications are restored.
Fiji Red Cross disaster co-ordinator Vuli Ngauna says it wants to get tarpaulins, food and blankets to the northern parts of Vanua Levu and the surrounding islands.
However, he says this will not happen until at least Wednesday.
Tomas is believed to have claimed one life on Friday, when a woman was swept out to sea.
Police say the 31 year old and her relatives were washed out at Namilamila Bay in Vanua Levu after a warning to stay away from the sea was issued.
NZ has plane on standby
The New Zealand Government has an Airforce Hercules on standby, to provide assistance to Fiji.
Foreign Affairs Minister Murray McCully says the plane will head to Fiji as soon as the weather conditions ease.
He says Fiji has requested international assistance and New Zealand is working closely with its regional disaster relief partners, France and Australia, on how to best respond.
Mr McCully says New Zealand stands ready to help Fiji, and that the ongoing political differences between the two countries will not inhibit the Government's response.
Meanwhile, Air New Zealand says it has resumed normal service to Fiji's international airport at Nadi.
French aid for Wallis and Futuna
Supplies are being flown to Wallis and Futuna after Cyclone Tomas hit the French territory, with winds up to 200km/h damaging homes at the weekend.
The military planes are flying from their base in New Caledonia carrying tarpaulins, chainsaws, generators and building materials.
About 25 experts will be brought in to help with the reconstruction.
