The United Nations said Friday it was adopting zero-Covid methods of delivering badly-needed food, fuel and clean water to Tonga as the volcano and tsunami disaster relief efforts finally get rolling.

UN agencies told reporters in Geneva that now Tonga's main runway had been cleared of ash following Saturday's eruption and the ports were opening, methods of bringing in aid that required no contact with locals were being taken up.

The remote Pacific island nation, home to around 100,000 people, recorded its only Covid-19 case in October and has imposed strict protocols to keep out the virus.

The UN's humanitarian agency OCHA said that incoming relief workers could face weeks in quarantine, so it was focusing on "no-contact delivery".

"There is a very strong message from the government that they will not have Covid coming into the country in this response," spokesman Jens Laerke said.

He said options included planes landing and being unloaded by local crew without anyone on board exiting the aircraft, ships being offloaded by cranes while the crew stayed on board, and helicopters flying out to ships moored offshore.

"These are these options that are all being looked at to ensure that we adhere fully to the protocols," he said.

– Drinking water a 'serious concern' –

Australia and New Zealand sent in the first humanitarian flights on Thursday, five days after the dual disaster cut off the remote kingdom.

World Health Organization spokesman Christian Lindmeier said there were fortunately few casualties and that health installations were operational.

"The most important health needs are the drinking water, food and apparently also fuel to get everything rolling," he said.

"As long as this can be brought in with the contactless efforts, this seems to be fine.

"It looks right now that emergency teams… (are) not necessary at this point."

Laerke said that access to safe drinking water was a "serious concern" for around 50,000 people — roughly half the population — while an estimated 60,000 had been affected by damage to the agricultural sector.

He said the depth of ash cloaking the Polynesian archipelago was worse than initially reported, at about 10 centimetres (four inches).

And while Tongans are cleaning ash away, "there are health concerns with that, because some of that may be toxic".

The World Food Programme said it was readying its resources to go in.

"It would appear all agricultural sectors have been badly affected, from crops to livestock and fisheries. Initial estimates show up to 12,000 agricultural families have been affected," said spokesman Tomson Phiri.

"Roughly 60 to 70 percent of livestock-rearing families are estimated to have been affected.

"It's likely most families could do nothing to protect or save their livestock from perishing and for those that survived, there may be very little grazing pasture and uncontaminated water supplies left."