It caused 10 cubic kilometres - the equivalent of 2.6 million Olympic-sized swimming pools - of seafloor to be displaced and destroyed the ecosystems around the volcano.

Project leader and marine geologist Kevin Mackay said the eruption reached record heights and the material it produced circulated the atmosphere for months.

"[It was] the first we've ever seen to break through into the mesosphere. It was like a shotgun blast directly to the sky," he said.

NIWA scientists also discovered currents of dense lava, volcanic ash and gases - called pyroclastic flows - travelled as far as 100 kilometres following the eruption.

This is the first time scientists have seen underwater pyroclastic flows of this magnitude.

 READ MORE

• Tsunami warning after volcanic eruption in Tonga, Cyclone Cody approaching
• Tonga eruption and tsunami: Jacinda Ardern gives update as officials determine what help New Zealand can provide
• Chinese debt a concern in Tonga's post-eruption rebuild
• New Zealand makes its first aid delivery to Tonga after Hercules lands with supplies

"They had enough power to flow uphill over huge ridges and then back down again," principal scientist Dr Emily Lane said.

The eruption was so violent the tsunami it generated travelled across the Pacific and world-wide, Lane said.

The non-profit organisation The Nippon Foundation worked with NIWA on the investigation.

"A huge proportion of the Earth's population live on the coast, which are already vulnerable to the impacts of climate change, sea level rise, and big storms. We need to further our understanding of the risks

from underwater volcanos so we can better prepare and protect future generations and their ecological environments," he said.

A project supported by The Nippon Foundation aims to map the world's oceans by the end of the decade.