As the death toll from Cyclone Winston continues to rise, there are fears of widespread disease outbreaks across Fiji due to the lack of clean water.
SBS News's Nastasya Tay, reporting from Fiji.
Aid agencies fear there could be widespread disease outbreaks in cyclone ravaged Fiji with clean water supplies cut off and thousands left homeless.
CARE Australia spokeswoman Sarah Boxall says thousands of people are sheltering in overcrowded evacuation centres and are at increased risk of diarrhoea from water-borne contamination, as well as typhoid and dengue.
Ms Boxall says it's a race against time to get immediate relief to those who have lost everything and ensure families can stay safe and healthy.
The official death toll rose rapidly on Monday as information began filtering back from hard-hit remote areas that bore the brunt of the category-five cyclone on Saturday: a storm believed to be the most powerful ever to hit Fiji.
Aid groups and government authorities are now racing to deal with a critical need for clean water, health supplies and emergency accommodation after power lines, roads, jetties and homes were destroyed and damaged across huge areas of Fiji's main island.
A 30-day state of emergency has been declared in Fiji and Prime Minister Frank Bainimarama has ordered emergency management officials to respond to the crisis as quickly as possible.
"There are Fijians out there who are without water, without a roof over their heads, without food and without essential services," Mr Bainimarama said.
Cyclone Winston struck on Saturday night with winds gusting up to 325km/h and waves rising up to 12m high.
The Fiji Broadcasting Corporation reported government officials have confirmed the death toll currently stands at 21 and four people are missing at sea.
Australian Red Cross aid worker Susan Slattery, based in the Fijian capital Suva, said the Fijian Red Cross was working to get tarpaulins, hygiene kits, kitchen kits and clothing to areas in the north and west of the main island.
Assessment teams are still working their way towards more remote regions, with blocked roads and wiped-out bridges slowing their progress, so the full extent of the damage is not yet known, Ms Slattery told AAP.
"Water is becoming an issue and will quickly become one of the top issues because if people don't have access to safe water they can quickly become sick," she said.
Power and communications are still knocked out across most of Fiji's main island and more than 8000 people remain in evacuation centres.
"I think everyone is starting to come to the realisation that this is going to be a very long road back," Ms Slattery said.
Australia's government has offered an initial $5 million assistance package to Fiji and the Australian Defence Force has also offered transport helicopters and two Orion surveillance aircraft to help with the relief effort.
UNICEF Australia has begun loading water and sanitation kits plus education and health supplies onto boats destined for cyclone-ravaged outer islands.
More than 60,000 people live in areas that were hit by destructive winds near the eye of Cyclone Winston, UNICEF Australia CEO Adrian Graham said.
More than 400,000 people, out of a population of around 900,000, are in areas affected by strong winds and rain.
Scheduled flights to and from Fiji have resumed with Virgin Australia, Air New Zealand and Fiji Airways flying, while Jetstar expects to resume flights on Tuesday.
UNICEF has set up a Fiji Children's Emergency Appeal to help children and families in need of urgent assistance.
The Australian Red Cross has also launched an appeal, with donations able to be made via its website.
Some 1300 Australians are believed to be in Fiji but that figure may far higher given up to 350,000 visit Fiji every year.
"The full impact of this disaster is still not known," Foreign Affairs Minister Julie Bishop said in a statement.
"We stand ready to provide further assistance to support Fiji's relief and recovery efforts."