A distress signal has been detected in a low-lying Tongan island following the volcanic eruption and tsunami, the United Nations said Tuesday as the first death was reported.

The UN Office for the Coordination of Humanitarian Affairs expressed concern for two islands, Mango and Fonoi, after surveillance flights confirmed "substantial property damage" from Saturday's eruption but there had been no contact with residents.

"An active distress beacon had been detected from Mango," the OCHA said. The island is home to more than 30 people, according to Tongan census figures.

The agency said extensive damage had been reported on the western beaches of Tonga's main island, Tongatapu, with several resorts and houses either destroyed or seriously damaged and two people were missing.

No details of the missing were released but the brother of a British woman who was swept away by the tsunami said his sister's body had been found.

Nick Eleini said the family was "devastated" that Angela Glover had died wile trying to rescue her dogs.

"Earlier today my family was sadly informed that the body of my sister Angela has been found," Eleini said in a statement.

Angela Glover, 50, and her husband lived in Tonga were she ran an animal welfare charity providing shelter and rehoming stray dogs.

A screen grab taken from a US weather satellite on January 15, 2022 shows the volcanic eruption that triggered a tsunami in Tonga
A screen grab taken from a US weather satellite on January 15, 2022 shows the volcanic eruption that triggered a tsunami in Tonga NOAA/GOES via AFP / Handout

"I understand that this terrible accident came about as they tried to rescue their dogs," Eleini said.

Three days after the eruption, information coming out of the Pacific archipelago is scarce with most communication links severed.

New Zealand MP Jenny Salesa, whose electorate has a large number of Tongan residents, said everyone was praying for the people of Tonga.

"They're really just worried about their safety," she told Radio New Zealand.

"It's basically not knowing whether they are well, not knowing whether any of their family members are missing or (have) been washed out to sea. It's basically that lack of communication that is worrying to people back here in New Zealand."

The New Zealand and Australian air forces conducted surveillance flights over Tonga on Monday and were preparing further flights to carry emergency supplies to the Pacific kingdom.

The OCHA said that while communication issues meant they could not fully assess the impact of the eruption and tsunami, there was concern about the contamination of drinking water and crops, and the need for safe water supplies.