GNS Science says Monday night's eruption of Mt Tongariro has blown open three new steam vents.

GNS vulcanologist Michael Rosenberg says photos taken on Tuesday afternoon of the Te Maari craters show steam is still coming out.

He says the images clearly show three new vents which must have been created during the eruption on Monday night.

Photo: GNS SCIENCE

Meanwhile, the Department of Conservation says one of the huts on the mountain was significantly damaged after the eruption.

DoC's Nic Peet says Ketetahi hut about 1.5km from the eruption's epicentre has been significantly damaged with holes in the roof, floor and bunks.

He also says boulders up to a metre in size landed on the track leading up to the hut, leaving impact craters.

Dr Peet says the Tongariro Alpine Crossing as well as four huts on the mountain will remain closed until further notice.

Photo: NZ Police

Could erupt again any time

Mt Tongariro has quieted since Monday's eruption but it could erupt again at any time. The eruption - Tongariro's first for 115 years - spread an ash cloud across the central North Island.

The eruption which occurred at 11.50pm forced some people to leave their homes and eyewitnesses reported loud explosions, bright flashes and plumes of smoke spewing from the crater. Ash coated nearby roads and ash fall has been reported - some as far away as Napier.

GNS Science says the eruption was accompanied by an hour-long earthquake with a magnitude of between 2.5 and 3.

It says when Tongariro last erupted in the 1890s there was a long series of eruptions and scientists are trying to decided if that pattern will be repeated.

It appears the eruption happened at Te Maari Craters, which are close to the Ketetahi Hot Springs on the northern side of the mountain.

More small eruptions wouldn't surprise

GNS Science vulcanologist Brad Scott says the eruption was a hydrothermal event, and a seismograph of the mountain shows no more energy has been released since then.

But Mr Scott says he wouldn't be surprised by more small hydrothermal eruptions.

He says activity has been seen at the mountain for the past two to three weeks and if that expands, there could be a magmatic event.

GNS Science says it has been recording small earthquakes under Tongariro for the past few weeks but it is unusual for a volcano to go from a dormant state to an active eruption so quickly.

Vulcanologist Michael Rosenberg says GNS Science had even issued a volcanic alert statement on Monday saying everything was quiet.

Eyewitness reports of event

A woman who lives near Mt Tongariro, Robyn Bennett, says she and her husband got out of the house as soon as the eruption happened.

Mrs Bennett says the view from outside was spectacular.

"It was just this huge mushroom," she says, "like you see with the atomic bomb - zooming up like that. And fireworks going off inside it like rocks firing out from all angles, and lightning going off."

David Bennett, who lives about five kilometres from the volcano, says the noise of the eruption initially sounded like the build-up of a strong wind, and when he looked out the window he also thought it was like fireworks.

Police say a member of the public phoned just before midnight, saying they could see an orange or yellow glow from the mountain.

The man described seeing flame-like explosions and a cloud of ash coming from "a new hole in the side of the mountain."

Truck driver Bryn Rodda was on the Desert Road just before midnight and he says the ash was so thick he had to slow down considerably as he passed through the cloud.

"I could see this big cloud - it looked like a fist, basically, at an angle across the sky," Mr Rodda told Morning Report, "and about the wrist section of the fist there was an orange ball of flash."

Alert level raised for White Island

The alert level for volcanic eruptions on White Island in the Bay of Plenty has been raised to two.

An eruption on Sunday followed a volcanic earthquake the weekend before and an increase in sulphur gases and the lake level.

The general manager of natural resource operations at the Bay of Plenty Regional Council, Warwick Murray, says GNS does not know whether the increased seismic activity is connected to the Tongariro eruption.

He says it follows two years of relative quiet at White Island.

Mr Murray says visitors to the island are being warned and there is an alert for aircraft.