Updated about 1 hour ago
GNS Science says Mt Tongariro has quieted since an eruption just before midnight on Monday night but it could erupt again at any time and is being closely monitored.
GNS vulcanologist Mark Rosenberg says they have observers en route to the mountain and they are monitoring earthquakes around the volcano.
The eruption - Tongariro's first for 115 years - spread an ash cloud across the central North Island and forced some people to leave their homes.
It occurred at 11.50pm, with eyewitnesses reporting 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.
A GNS Science webcam image of Mt Tongariro at midnight on Monday night.
PHOTO: GNS SCIENCE
Police say ash and rock erupted from the volcano out to a radius of about one kilometre.
Volcano scientists have been holding a meeting in Taupo on Tuesday morning to determine whether the mountain is likely to erupt again.
GNS Science vulcanologist Steve Sherburn says scientists have flown over Tongariro and seen a plume of steam rising from the mountain but otherwise the eruption appears to have subsided.
Mr Sherburn says the eruption was accompanied by an hour-long earthquake with a magnitude of between 2.5 and 3.
He 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 Mari Craters, which are close to the Ketetahi Hot Springs on the northern side of the mountain.
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.
Listen to interview with Michael Rosenberg on Morning Report
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."
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.
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