The massive underwater Eruption near Tonga in the Pacific was probably the biggest volcanic explosion in the world in 30 years, according to an Australian earth scientist.

Although scientific data which will confirm the size of the eruption is still to be finalised, Australian National University volcano expert Richard Arculus said the Hunga-Tonga-Hunga-Ha'apai eruption would likely sit on a world top 10, alongside the eruptions of Krakatoa in 1883 and Mount Pinatubo in 1991.

Professor Arculus expected more Hunga-Tonga-Hunga-Ha'apai eruptions over the coming days and weeks, but he said explosions should be smaller than what appeared to be a one-in-1000 year blast on Saturday, which sent a plume of ash, gas and steam at least 20 kilometres into the air.

LIVE UPDATES: Australian Open day 1

READ MORE: NSW cases drop but hospitalisations rise

Similar to seismologists who use a Richter scale to measure earthquakes, volcanologists have a Volcanic Explosivity Index (VEI) to gauge of the explosiveness of volcanic eruptions.

"This would be up there at around probably a five or six," Professor Arculus said.

"So it's pretty big.

"On a scale of bangs, it's up there with Krakatoa."

The maximum VEI is 8, which humans have never witnessed.

An eruption's VEI is based on the amount of material thrown out, the time over which that material was thrown out, and the eruption height column.

Some reports today suggested the plume could have climbed as high as 30 kilometres.

READ MORE: Tonga calls for 'immediate assistance' after volcano, tsunami

READ MORE: Search for girl, 9, in Blue Mountains enters fourth day

Professor Arculus described the eruption as "spectacular", an impression amplified because it had been captured on satellite imagery, which showed a massive ash cloud and shockwaves spreading from the eruption.

The explosion could be heard in Alaska, over 9000 kilometres away, and the sound wave spread out across the Pacific at 1200 kilometres an hour, Professor Arculus said.

The eruption caused a Tsunami on Tongatapu, with waves flowing onto coastal roads and flooding properties.

Tsunami warnings were issued on Australia's east coast, parts of New Zealand, Japan, the United States and Canada.

https://twitter.com/NiwaWeather/status/1482259999724535809?ref_src=twsrc%5Etfwhttps://twitter.com/USGSVolcanoes/status/1482805903124414464

Professor Arculus explained how the tsunami was not generated by the explosion of the volcano, but instead a more complex combination of tremendous forces at play.

He called it a "rebound effect of the ocean" because an underwater mountain had effectively been "vaporised".

Measuring around 1800m high and 20km wide, the Hunga-Tonga-Hunga-Ha'apai volcano is hidden under the water.

"The reason for the tsunami is not the explosion itself," he said.

"If you make a big hole in the crust, and throw out so much material, you are essentially vaporising a mountain.

"Then the sea rushes in to fill the cavity, and bounces back out again.

"So it's a kind of rebound effect for the ocean … and that spreads all the way, circumferentially, radially out, across the ocean.

"So that's the source of the tsunami."

When Krakatoa blew, killing 36,000 people, it was this rebound effect which generated a catastrophic tsunami 37-metres high.

https://twitter.com/WeatherWatchNZ/status/1482876643819810816

People on Tonga today described their country as looking like a moonscape as they began the task of cleaning up from the tsunami waves and ashfall caused by the eruption.

Communications with the island nation remained limited after the internet was cut soon after the eruption on Saturday evening.

There were no reports of injuries or deaths, although concerns remained for the fate of people on some of the smaller islands near the volcano.

With AP