A volcanologist has called White Island a "disaster waiting to happen" as questions mount about why tourists were ever allowed on the famously rowdy volcano.

It comes as the mother of one American tourist, who was badly burned in yesterday's eruption, said she was "livid" that visitors could step foot on the island.

Emeritus Professor Ray Cas, of Australia's Monash University, said he'd visited the island twice and always thought it was too dangerous to allow daily tour groups onto.

Cas noted that many of the island's eruptions had been hydrothermal, resulting from superheating of the geothermal waters by molten rock or magma within the volcano.

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"Most of the material that is explosively ejected is hydrothermally altered country rock for around the geothermal vents, but sometimes the ejected material includes fragments of fresh magma," he said.

"In addition to the hot rock fragments and fragments of magma, large volumes of volcanic gas and superheated steam are released which produce a hot plume of gas and rock that rises above the vent, sometimes to heights of thousands of metres."

Another overseas scientist, Dr Jessica Johnson of the University of East Anglia in the UK, said the eruption was "unfortunate but not completely unexpected".

"Levels of activity at White Island/Whakaari have been relatively high since September, and even more elevated over the last couple of weeks, with increased numbers of small earthquakes and more volcanic gas detected than usual," she said.

"As a consequence, the volcanic alert level was raised. Similar eruptions have happened over the last 100 years or so."

She noted that even though the alert level was raised, it was still very difficult to forecast exactly what would happen at volcanoes.

"White Island/Whakaari is an andesitic stratovolcano, which means that it can have lots of different types of eruptions. It also has a water-filled crater lake," she said.

"When water reacts with hot rock or magma, it can create explosions, and therefore, can make eruptions even more difficult to forecast."

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Earlier, University of Auckland volcanologist Professor Shane Cronin also pointed out that unheralded eruptions from volcanoes such as White Island could be expected at any time.

Under the GeoNet-managed NZ Volcanic Alert Level system, ranging from zero to five, the volcano had recently been rated level 2, indicating "moderate to heightened volcanic unrest".

That had been raised in response to increasing amounts of sulphur dioxide gas, along with volcanic tremors – both which can signal rising magma deep in the volcano.

It was ultimately up to operators to decide whether to take visitors to the privately-owned island, with access controlled through permits.

In 2017, the Government and the Bay of Plenty Civil Defence Emergency Management Group signed a memorandum of understanding setting out responsibilities around readiness and response to an eruption.

One Whakatane company, White Island Tours, stated on its website that it operated through varying alert levels, but added that "passengers should be aware that there is always a risk of eruptive activity regardless of the alert level".