Scientists say there could be more eruptions at Mt Tongariro, after last night's explosion - the first there for more than a century - spewed rock from the volcano in a 1km radius.
Residents were this morning urged to check their water supply after last night's event created an ash cloud that forced the cancellation of flights and closure of roads. The cloud is drifting east and ash has fallen as far east as Napier.
The Department of Conservation has evacuated huts on the mountain. There have been no reports of injuries or damage but residents with respiratory problems have been told to consider staying inside.
The eruption was reported to police just before midnight by a member of the public who reported seeing explosions on the northern face of the mountain.
The witness told police the eruption had created ``a new hole in the side of the mountain''.
New Zealand Herald reporter Jamie Morton said the ash this morning on SH 46 at Lake Rotoaira, just north of Tongariro, was a ``thick, clay-like mud'', about half a centimetre thick.
``It's just carpeted everything, all the fields, cars, trees - the whole landscape looks quite murky and grey,'' he said.
``Roofs in this area, they're all absolutely coated in this ash.''
GNS volcanologist Brad Scott said the eruption began from Te Mari crater at the north end of Mt Tongariro at 11.50pm.
An earthquake lasting about five minutes accompanied the eruption and residents reported hearing explosions.
``When they went outside to have a look they saw the volcano starting to erupt. They saw incandescent blocks - glowing hot blocks - and they saw an eruption column being developed and within about five minutes they were experiencing a light local ash fall,'' Mr Scott told Radio New Zealand.
David Bennett, who lives on the southern shores of Lake Rotoaira, about 6km from the eruption, said he saw ash spewing from Mt Tongariro.
''[I saw] just a big cloud heading straight up from the crater, thunder and lightning from in the cloud, and then there was a smaller cloud drifting northward, lasted I suppose about half an hour and then the dust cloud headed over towards the east.''
He knew what was happening straight away and kicked an evacuation plan into action.
While his wife checked on his parents, he went and checked his neighbours were awake.
``Because wind was blowing the other way, we just waited here.''
When he got closer to the mountain this morning he said it looked like another crater had blown open.
Anne Lambert, who owns the Rainbow Motel, about 4km from Turangi, said she got out of bed after hearing a loud noise about midnight.
``I heard this loud rumble, like a big lot of trucks coming by, but it didn't go away.''
As she looked out from a second-storey deck she could see a large plume of smoke sitting in the middle of the volcano, with sparks coming out of it.
``I couldn't believe what I was seeing.''
The explosions were heard as far away as Gisborne.
One Hexton resident said: ``The pheasants started crowing away and I thought an earthquake was coming next. They always do that when there is going to be an earthquake.
``Then the dogs started howling and I thought it might be a big one. I heard three massive explosions. It was boom, boom, boom, and then it sounded like a stock rolling flat-out down a hill. It was unbelievable.''
Mr Scott said GNS had been aware of activity at Mt Tongariro for a few weeks, ``but to be honest we didn't see anything in the latest data up until last night that indicated it was ready to erupt'', Mr Scott said.
And he said there was likely to be further activity.
``There's not showing any escalation - the earthquake activity hasn't increased or anything like that - but we would probably anticipate some more activity now that the craters have broken through.''
Mr Scott told a press conference this morning the eruption was driven by steam, rather than molten lava rising to the surface.
``We've had a small-scale volcanic eruption. It appears to be driven in the hydrothermal rather than the magmatic process, there's been an ash plume, there's been ash-fall down wind.''
Mr Scott described last night's eruption as ``small scale'', but he said he would not be surprised if there were more similar eruptions to come.
Brent Crowe of Bay of Plenty police told the press conference ash and rock was ejected over a 1km radius.
Police closed State Highway 1 and State Highway 46 as a precautionary matter overnight but they had since reopened.
The police focus remained on public safety and he said they expected the situation to stabilise.
The wider community's health was not currently at risk, and earlier warnings to remain indoors with closed doors and windows had been lifted.
``At this time the only risk is minimal and would only be to people in the local vicinity of the eruption who have a predisposition to respiratory issues,'' police said in a statement.
Locals in the area were urged to check their water to supply to ensure it had not been contaminated.
Nic Peet of the Department of Conservation said DOC would be taking a precautionary approach to opening facilities around Tongariro.
The Tongariro Crossing and its four huts were currently closed.
Mr Peet said there was no records kept of how many people were up on Mt Tongariro last night.
Three people were this morning evacuated from the Mangatepopo Hut, he said.
Air New Zealand has cancelled all flights to and from Hawkes Bay airport but skifields on neighbouring Mt Ruapehu are operating as normal.
Federated Farmers said initial reports indicated the eruption had little impact on farm pasture or stock drinking water.
int agency incident management centre has been established at the Whakapapa Department of Conservation Visitor Centre.
Mt Tongariro last erupted between 1896 and 1897.