The Ministry of Foreign Affairs and Trade has confirmed two deaths so far following the volcanic eruption and tsunami in Tonga on Saturday.
The Hunga Tonga-Hunga Ha'apai volcano, which erupted on Saturday, was about 65km north of Tonga's capital Nuku'alofa.
There is now a huge clean-up operation in the town, which has been blanketed in thick volcanic dust.
Serious damage has been reported from the west coast of Tongatapu and a state of emergency has been declared.
The Ministry of Foreign Affairs and Trade has confirmed two deaths so far, but Fiji-based United Nations co-ordinator Jonathan Veitch said there were still areas that had not been contacted.
One of those fatalities was British national Angela Glover, who was reported by her family to have been killed by the tsunami, he said.
Glover is thought to have died trying to rescue her dogs at the animal charity she ran.
Veitch told RNZ full information from some islands - such as the Ha'apai group - was not available.
"We know that the Tonga Navy has gone there and we expect to hear back soon."
The communication situation was "absolutely terrible".
"I have worked in a lot of emergencies but this is one of the hardest in terms of communicating and trying to get information from there. With the severing of the cable that comes from Fiji they're just cut off completely. We're relying 100 percent on satellite phones.
"We've been discussing with New Zealand and Australia and UN colleagues ... and we hope to have this [cable] back up and running relatively soon, but it's been a bit of a struggle."
It had been "a lot more difficult" than regular operations, Veitch said.
One of the biggest concerns in the crisis was clean water, he said.
"I think one of the first things that can be done is if those aircraft or those ships that both New Zealand and Australia have offered can provide bottled drinking water. That's a very small, short-term solution.
"We need to ensure that the desalination plants are functioning well and properly ... and we need to send a lot of testing kits and other material over there so people can treat their own water, because as you know, the vast majority of the population in Tonga is reliant on rainwater, and with the ash as it currently is, it has been a bit acidic, so we're not sure of the quality of the water right now."
Another issue was access.
"Tonga is one of the few lucky countries in the world that hasn't had Covid ... so we'll have to operate rather remotely. So we'll be supporting the government to do the implementation and then working very much through local organisations."
For those in Tonga who were cut off, Veitch said the main message was "everybody is working day and night on this. We are putting our supplies together. We are ready to move. We have teams on the ground. We are coming up with cash and other supply solutions ... so help is on its way".
Earlier today, New Zealand's Acting High Commissioner for Tonga, Peter Lund, told Tagata Pasifika that there were unconfirmed reports of up to three deaths following the weekend's eruption of Hunga Tonga-Hunga Ha'apai and the resulting tsunami.
"I understand, talking to our police adviser this morning, unconfirmed reports of up to three fatalities, but that is not yet confirmed."
Speaking to the news outlet from Nuku'alofa via satellite phone, Lund said there was a huge clean-up operation underway in the area after the town had been blanketed in a thick film of volcanic dust.
Meanwhile, the United Nations say a distress signal has been detected in an isolated group of islands in the Tonga archipelago following Saturday's volcanic eruption and tsunami, prompting particular concern for its inhabitants.
The UN Office for the Co-ordination of Humanitarian Affairs said there had been no contact from the Ha'apai group of islands and there was "particular concern" about two small low-lying islands - Fonoi and Mango, where an active distress beacon had been detected.
According to the Tonga government, 36 people live on Mango and 69 on Fonoi.
- additional reporting RNZ