Homes have been destroyed, government buildings flattened and churches devastated following the severe tropical cyclone Gita.
The full scale of destruction is beginning to emerge from Tonga in the aftermath of the storm with reports of around 50,000 people or almost 70 per cent of the country's population having been affected, a third of whom are children.
Oxfam's country director for Tonga, Jane Foster, said the agency's main focus now is to work with local partners to find out exactly where the most urgent needs are.
She said water supplies across the main island of Tongatapu had sustained significant damage, which brings with it the risk of further outbreaks of waterborne diseases.
"The impact of this severe storm will be felt on many people's livelihoods for a long time to come.
"We also have grave concerns for the immediate threat from damage sustained to water supplies as the risk of contamination is high.
"There is a real risk of a second disaster from water and mosquito-borne illnesses like dengue," she said.
"Oxfam's main relief efforts will focus on our area of expertise: providing safe water for people, as well as sanitation supplies and public health support to help prevent the spread of water-borne diseases."
Cyclone Gita made landfall in the southern part of Fiji last night as wind gusts reached 190km/h and the storm was upgraded to Category-5 status.
But Fiji Red Cross secretary General Filipe Nainoca said the community of outer islands that was feared for seems to have "dodged a bullet".
"There is not as much damage as we initially anticipated so that is good news for us."
He said Ono-i-lau, Oqea and Vatoa Islands were affected by the storm, which house around 1000 people.
"Some of the houses had their roofs taken off in one particular area, and we are waiting to get some more details from the others," he said.
"The number of houses that are completely destroyed, is not many. It looks like we dodged a bullet."
Nainoca said as soon as the weather is safe relief workers would sail to the islands.
Foster said some of these islands are extremely remote and hard to reach in the best of times.
"We hope that preparedness plans and evacuation centres keep people safe until support reaches them."
New Zealanders wanting to support people affected by Cyclone Gita are urged to ensure their generosity has the biggest impact by sending cash, not goods, Foster said.
"In a disaster, many people are often moved to send goods they think can help. But for the cost of shipping nine litres of bottled water from New Zealand to Nuku'alofa, Oxfam can produce over 16,200 litres of safe drinking water in Tonga."
Donations to support Oxfam's emergency responses in the Pacific and around the world can be made online at http://oxfam.org.nz/drf or by calling 0800 600 700.