The Ministry of Civil Defence and Emergency Management says the threat from Mt Tongariro has passed, after it erupted last night.
However, it was too early to predict the next series of events, and GNS Science expected heightened activity could continue for several weeks.
This afternoon the Ministry of Civil Defence and Emergency Management cancelled its national advisory, after latest information from GNS Science said eruption activity had subsided.
GNS Science also downgraded its aviation colour code from red to orange shortly after midday today.
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".
GeoNet this morning said the plume from the eruption last night was steam-driven, coming from the hydrothermal system rather than from new molten lava rising to the surface.
It shot almost 7000m into the air, "which is not insignificant", Civil Aviation Authority manager of meteorology Peter Lechner said.
The plume was now sitting over the volcano and to the east towards Hawkes Bay.
"So that block of plume is just quietly drifting away to the east where it will be right off the coast later this evening," Mr Lechner said.
GeoNet spokesman John Callan said the seismic activity had died away, as well as the steam plume.
The downgrade of the colour code - which is used to alert aviation operators to activity around a particular volcano - was because there was less ash in the air and the plume was much smaller.
However, it was difficult to predict what would happen next.
"It is too early to predict the next series of events, but we expect heightened activity may continue for several weeks. There are likely to be specific signals of future magma movement beneath the volcano and we continue to monitor the situation through the GeoNet volcano-seismic network of instruments," GeoNet said in a statement.
"As with any volcano, an eruption could occur at Tongariro at any time with little or no warning and there is an elevated level of risk, particularly on the northern slopes and valleys 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.
David Bennett, who lives on the southern shores of Lake Rotoaira, about 6km from the eruption, said he saw ash spewing from Mt Tongariro about 11.50pm.
"[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 reportedly 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 three men walked out of the Mangatepopo Hut this morning, but no one was staying in the other three huts on the mountain.
Police, DOC and search and rescue volunteers went into the Mt Tongariro area to check the local tracks and huts this morning.
The Tongariro Alpine Crossing track remained closed and would be reviewed on a daily basis.
Walks around Mt Ruapehu were unaffected and ski fields on Mt Ruapehu were open.
Mt Tongariro last erupted between 1896 and 1897.
APNZ
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