GNS volcanologist Graham Leonard says White Island/ Whakaari erupted suddenly.
Speaking to media on Tuesday evening, Leonard said the explosion occurred "immediately with no warning".
"[The island] has large steam and hydrothermal system near the vent which is driving it below. So the eruption like we saw yesterday can occur immediately with no warning because it's an immediate destabilisation of that environment."
He says there was no real indicator of the eruption that he could see.
"It is clearly able to erupt with no precursory signals."
The volcano was sitting at alert level two when it erupted on Monday at 2:11pm, killing five people and injuring 31.
"Level one and two are terms of unrest," said Leonard.
"Three, four and five are small, medium and large eruptions," he continued.
Volcanic alert bulletins put the Whakaari/White Island eruption at level four after it exploded on Monday.
The eruption was the largest in the island's recent history and police presume anyone who has not been rescued from the island is unlikely to be alive.
Prime Minister Jacinda Ardern says there will be an inquiry into the health and safety procedures around tours of the island.
