University of Otago geologist Dr Marco Brenna was part of a team that camped in the area for four nights.
They travelled to Hunga-Tonga-Hunga-Ha’apai to survey and take samples of the cone from 2014-15 eruptions that created land between two rocky islands.
Results indicated there was a shallow magmatic system, about 5km-6km deep, that was slowly being recharged and inflating, Dr Brenna said.
"There was, however, no obvious sign that such a large eruption was impending."
Nothing remains of the landscape where the party from the University of Auckland and a film crew camped.
The volcano had blown several times since 1900 — in 1912, 1937, 1988, 2009 and 2014.
Auckland University professor of volcanology Shane Cronin said early data suggested Saturday’s eruption was the largest the world had seen since Mt Pinatubo in the Philippines 30 years ago, RNZ reported.
Prof Cronin said scenes on the ground would have appeared apocalyptic: ash clouds blotting out the sun, thunderclaps of booming shockwaves and thousands of lightning strikes.
The eruption, about 65km north of Tonga’s capital, Nuku’alofa, sent thick ash and steam 20km into the air.
Back in 2015, on Hunga-Ha’apai, the scientists found evidence of previous large explosive events that occurred about 900 years ago, Dr Brenna said.
The scientists took samples from the older edifice and deposits from the eruptions from 900 years ago, and deposits from the 2009 eruption.
Dr Brenna said most of what was there in 2015 had been obliterated.
