Tonga has been virtually cut off from the rest of the world after a massive volcanic eruption that crippled communications with the Pacific island nation, and experts warned internet connection may not be fully restored for weeks.

It is two days since the Hunga Tonga-Hunga Haa'pai volcano exploded, cloaking Tonga in ash, triggering a Pacific-wide tsunami and releasing shock waves that wrapped around the globe.

But still neighbouring countries and international agencies are clambering to try to grasp the scale of the damage, with New Zealand's leader Jacinda Ardern saying yesterday that it is believed to be "significant".

Wellington and Canberra scrambled reconnaissance planes in an attempt to get a sense of the damage from the air today, with both also putting C-130 military transport aircraft on standby to drop emergency supplies or to land if runways are deemed operational.

What is known is that the volcanic blast on Saturday seriously damaged the ash-covered capital Nuku'alofa and severed an undersea communications cable - which could take two weeks to restore.

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The eruption was recorded around the world and heard as far away as Alaska, triggering a tsunami that flooded Pacific coastlines from Japan to the United States.

"We know water is an immediate need," Ms Ardern told reporters.

She added New Zealand was relying on satellite phones to communicate with the island nation that is home to some 100,000 people.

The reconnaissance flights would help to advise Tonga's government of the scale of the volcanic and tsunami damage and help to identify aid needs, Ms Ardern added.

The premier, who has spoken to the New Zealand embassy in Tonga, has described how boats and "large boulders" washed ashore north of Nuku'alofa.

Wellington's defence minister said he understood the island nation had managed to restore power in "large parts" of the city.

A handout image captured by NOAA's GOES-17 satellite and made available by the Regional and Mesoscale Meteorology Branch of National Oceanic and Atmospheric Administration shows the volcano blast

Tonga's internet may remain down for two weeks, Southern Cross Cable Network's networks director Dean Veverka said.

"We're getting sketchy information but it looks like the cable has been cut," he said.

"It could take up to two weeks to get it repaired. The nearest cable-laying vessel is in Port Moresby," he added, referring to the Papua New Guinea capital, more than 4,000km from Tonga.

Southern Cross is assisting Tonga Cable Limited which owns the 872km cable linking the island nation with Fiji - and from there to the rest of the world.

Initially it was believed the fault was due to power failure following the powerful eruption.

But further testing once power was restored indicated a break in the cable.

Tonga was previously isolated for two weeks in 2019 when a ship's anchor cut the cable and a small, locally operated satellite service was set up to allow minimal contact with the outside world.