More volcanic upheavals likely to follow, warns Aussie expert

But it is still "not a very predictable" process as to when it would explode, especially given the lack of data currently.

"At the moment, we actually do not have a lot of data. Most of the stations on the volcano would have been destroyed during the day. We have satellite data, but any sort of other measurement needs to be taken on site, which at the moment is not easy, basically impossible to travel there. Without data, it's really difficult to assess what sort of explosivity index can be assigned to the volcano or how much mess was in place."

Nebel said tsunamis were the biggest hazards of such underwater eruptions.

He said Saturday's disaster had been caused by the "displacement of water mass through injection of material from the deep Earth's interior into the ocean and the possible sinking of the volcano due to lava evacuation from underneath."

He also added that during the eruption, "lava burst into tiny particles that formed volcanic ash, and the interaction of lava with cold seawater likely intensified the plumes of ash."

"This ash is also causing a fair bit of air traffic disruption," he said. "The ash is actually tiny particles of rock, it's not like the ash out of a wood fire."

Nebel said the priority for authorities must now be to help those who have been hardest hit by the eruption.

One such priority will be restoring Tonga's undersea communications cable that was severed during the eruption. It is believed the repairs could take more time, which means the inhabitants of the island country are essentially cut off from communication with the outside world.