The Hunga Tonga-Hunga Ha'apai eruption on Saturday - which could be the world's most powerful blast in three decades - caused a tsunami across the Pacific and blanketed the Tongan capital Nuku'alofa in thick ash.

While much of Tonga's power has been restored, communication in and out of the remote Pacific nation remains limited.

The reconnaissance flights are due to report back late on Monday, providing a greater understanding of the natural disaster's impact.

The volcano sent ash around 20km into the sky in Saturday's spectacular eruption, but it is likely the tsunami of around 80cm hitting Tongan shores generated the most damage.

The United Nations has also confirmed damage to buildings and infrastructure, as well as concerns over access to fresh water.

New Zealand Prime Minister Jacinda Ardern said boulders and boats had washed ashore on Tongatapu, the largest island and home to the capital, Nuku'alofa, around 65km south of the volcano.

"Seeing some of those waves come in and peeling back fencelines and structures, you can see the force of those surges," she said.

"Everyone just wants to establish how wide scale that impact has been ... we want to be in Tonga and on the ground as soon as we are possibly able to be."

In a Facebook post on Monday evening the New Zealand High Commission in Tonga said "significant damage" had been reported along the western coast of Tongatapu, including to resorts and the waterfront of northern Nuka'alofa.

Reports from social media paint a troubling picture.

A public appeal has been made on Facebook for English woman Angela Glover, whose husband James believes she was washed away in the tsunami.

Another Facebook post made by the Ha'atafu Beach Resort said it and the Kanokupolu village and had been "completely wiped out".

"We will celebrate the small wins and gift of life from God for everyone and re-evaluate where we go from here as a family and a business," the post concluded.

New Zealand officials said internal communications and "80 per cent" of power had been restored, but the crucial communications link to the outside world had been severed.

Tonga Cable Ltd, which owns the 872km undersea cable linking Tonga to Fiji, claimed the break would need to be investigated and repaired, which may take weeks.

Tonga, with a population of 105,000, is comprised of 169 islands, a few dozen of which are inhabited.

NZ Pacific Peoples Minister Aupito William Sio said key members of Tonga's royal family were safe and well.

Volcanic ash in the air and on the runway in Tonga prevented an aerial mission in the immediate aftermath of the blast.

Just prior to 9am NZDT on Monday, a New Zealand Defence Force Orion aircraft took off from Auckland's Whenuapai base on a mission to investigate further, with an Australian P-8 Poseidon also leaving Brisbane on a similar mission.

"It's looking to provide an assessment of the outer islands in particular," Ms Ardern said.

New Zealand will follow the reconnaissance mission with flights from the heavy-duty Hercules C-130 to drop any needed provisions.

University of Auckland volcanologist Shane Cronin said the eruption was likely to be the world's biggest since the 1991 eruption of the Philippines' Mount Pinatubo.

"Our research into these earlier eruptions suggests this is one of the massive explosions the (Hunga Tonga-Hunga Ha'apai) volcano is capable of producing roughly every thousand years," he wrote in The Conversation.

"We found evidence of two huge past eruptions ... and then used radiocarbon dates to show that big caldera eruptions occur about ever 1000 years, with the last one at AD1100."

Australian Associated Press