The eruption caused internet and phone lines to go down on Saturday evening, leaving the 105,000 residents on the islands virtually uncontactable.

Initial reports suggest there haven't been mass casualties but Australian police have visited beaches and found significant damage with "houses thrown around", Australia's Minister for the Pacific Zed Seselja said.

British woman 'missing'

There are reports that a British woman named Angela Glover is missing after being swept away.

The 50-year-old animal charity worker has not been seen since her coastal home was hit by the waves. Her husband James is reported to have managed to hold on to a tree, but Ms Glover and their dogs disappeared.

Her brother Nick Eleini has fears for her safety since it has been more than 48 hours since she went missing.

"The tsunami swept both of them away, Angela and James. James was able to hold onto a tree after quite some period of time," he told broadcaster TZNZ on an emotional video call.

"It's been 48 hours, I'm not holding out much hope. I think it will be a body retrieval rather than finding her, but I'm hoping… it's all I can do."

Mr Eleini, who lives in Sydney, said he was en route to the UK to be with his mother because he is now "her only biological child".

"She really needs me," he said.

Breaking down in tears, he said he and his wife "adored" his sister and described her as a "lovely girl".

'Wait is excrutiating'

Mr Eleini told The Guardian that Angela and James had been housesitting a property on the west coast of the island of Tongatapu, Tonga's main island.

"James went back to their proper house on the south coast of the island, but Angela didn't turn up. James contacted the police and the British embassy there, where he was able to notify us of what happened," he explained.

Mr Eleini said the search for his sister is ongoing but the wait is "excruciating", adding: "I can't even believe the words are coming out of my mouth, to be honest."

He said that Angela and her husband had been living in Tonga since 2015, and she had formerly worked in advertising in London before starting an animal rescue shelter in Tonga called TAWS, where she is listed on the charity's website as vice president.

She is "popular" and loved in the community, he said, adding she has a passion for travel and "really embraced" the Tongan way of life.

Communities 'wiped out'

Meanwhile, two people are known to have drowned off a beach in Peru due to unusually high waves following the eruption.

There are reports that communities on Tonga's western coast have been badly affected.

The owners the Ha'atafu Beach Resort, on the Hihifo peninsula, 21 km (13 miles) west of the capital Nuku'alofa, said on Facebook it had been "completely wiped out".

Australia's foreign ministry and New Zealand's military said they had each sent a surveillance flight on Monday morning to Tonga to assess the extent of the damage.

Australian Prime Minister Scott Morrison has pledged to provide support for Tonga as early as possible but said the huge ash cloud covering the island had hampered relief efforts.

"There's been a lot of challenges there with the ash cloud and the disruption to communications and so we are working together to get as much support to Tonga as we possibly can," he told radio station 2GB.

New Zealand Prime Minister Jacinda Ardern said communications are limited and so contact has not been made with coastal areas beyond the Tongan capital Nuku'alofa.

No news from coastal areas

"Nuku'alofa is "covered in thick plumes of volcanic dust but otherwise conditions are calm and stable," she said. "We have not yet received news from other coastal areas."

Ms Ardern later said the main undersea communications cable was affected, most likely because of the loss of power.

However, she also said power was being restored in some areas on the islands and local mobile phones were slowly starting to connect to networks.

Devastation 'could be immense'

The International Federation of Red Cross said it was mobilising its regional network to respond to what it called the worst volcanic eruptions the Pacific has experienced in decades.

"From what little updates we have, the scale of the devastation could be immense, especially for outer lying islands," said Katie Greenwood, IFRC's pacific head of delegation.

Satellite images captured the volcanic eruption on Saturday as the explosion sent plumes of smoke into the air, triggering warnings of 1.2-metre tsunami waves and evacuation orders on several Pacific islands.

Sanya Ruggiero, a communications worker in the Fijian capital Suva, described how the eruption left his "entire house shaking", despite being around 750km away from Tonga.

He said: "My doors, windows were all rattling like hell. And mine was not even as bad as others.

"Hundreds of people ran out of their homes."