GNS Science said volcanic ash could be damaging and disruptive, but it was fortunate the ashfall was relatively light and mostly confined to the Tongariro National Park and Desert Road area.
Duty volcanologist Michael Rosenberg said the volcanic plume was white by the time of an observation flight over Tongariro this morning, which meant there was very little or no ash in it.
It was difficult to see the plume this afternoon due to the "terrible'' weather, but the plume was now mainly steam, and the ash hazard was considered to be low.
"There's no ash coming out at the moment and there hasn't been since early this morning,'' Mr Rosenberg said.
"Obviously with further distance from the volcano it just disperses more and more, and gradually just becomes part of the atmosphere.''
Mr Rosenberg said the ash cloud would have contained fragments of volcanic ash - tiny particles of fragmented rock about 2mm in diameter - as well as droplets of water and probably acid.
The cloud would also have contained volcanic gases like carbon dioxide and sulfur dioxide.
Mr Rosenberg said the ash cloud after the 1995 Ruapehu eruptions contained toxic chemicals like fluorine.
"Fluorine is very toxic to stock and if there was enough in the ash accumulating on people's roofs where they collect drinking water, that would also be a significant health hazard,'' he said.
But it was not yet known if that was the case with the current eruption.
"We haven't yet been able to analyse any of the ash samples that we've had passed to us so we don't know if there are those chemicals in this ash.''
Scientists were hoping to know more about the components of the ash by tomorrow, while a full chemical analysis could take a few days.
Civil Aviation Authority meteorology manager 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,'' he said.
WeatherWatch said light north to northwest winds were expected tomorrow, followed by light west to southwest flows on Thursday.
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.
Orange means the volcano is experiencing heightened unrest with increased likelihood of an eruption, while red means eruption is forecast to be imminent.
GNS Science said there was the potential for more Tongariro eruptions to produce more ash, and people should be aware of the hazards.
Volcanic ash was made of fine particles of volcanic rock, less than 2mm in diameter, and was very different to ash from a fire.
It was usually non-toxic to people, but could be irritating - mostly to the eyes and lungs - and could exacerbate existing respiratory conditions.
Volcanic ash generally did not produce long-term health effects.
Civil Defence earlier warned people in affected areas to stay indoors as volcanic ash was a health hazard - especially to people who suffered from breathing difficulties.
"If people are caught in volcanic ashfalls, it's important they wear a dust mask or a handkerchief over their nose and mouth,'' a spokesman said.
People caught outside in ash were advised to seek shelter and to close all windows and doors inside.
People who relied on rainwater for drinking were advised to disconnect their pipes if ash had already fallen in their area, or if ash from further activity was likely to fall in their area.
Drainpipes and downspouts from gutters should be disconnected to stop drains clogging, but allow ash and water to empty from gutters onto the ground.
No talk of volcano danger: local
Rotoaira local David Bennett was concerned by the lack of communication between the authorities and National Park residents when Mt Tongariro erupted about 4km from his home last night.
He said locals had to contact one another to make sure everyone was all right, and they even set up two checkpoints on State Highway 46 to ensure all were accounted for.
"You've got to take some responsibility, and we had a plan," he told APNZ.
"What really annoyed me this morning when I looked at the TV, there was all these departments making bold statements but none of them were on the ground last night."
Mr Bennett was woken shortly before midnight by what he thought was wind.
"It turned out to be a little bit more than the wind when I looked out the window and Mt Tongariro was ... sending up a big plume of ash and steam, lots of thunder and lightning inside it, which is normal with a volcano, as I've seen when Ruapehu erupted. It was quite awe inspiring."
He said he was not concerned about further eruptions.
"The volcano will do what the volcano is going to do and, as a scientist said this morning, it's a bit like a bottle of Coke: you open the lid and it lets a bit of the pressure off and it'll settle down. If it doesn't, well, it's going to make some more bangs and we just go from there."
A layer of ash is clearly visible along the highways in the vicinity of the volcano, which is turning to a substance like wet cement as the rain falls over National Park.
The ash in the air causes a gritty feeling in one's mouth, and there is the distinctive scent of sulphur in the air.
Assessment planned
Mr Peat said DOC and GNS Science would be working on a risk assessment around public access to Mt Tongariro over the next 24 to 72 hours.
GNS volcanologist Brad Scott said scientists had collected samples from the site which would be analysed to see if was a steam-driven eruption or something more sinister.
"If it is purely a steam-driven, hydrothermal-type thing it's unlikely to do much more because once the initial pressure drop has occurred you'll get smaller activity.
"If it is being driven by a long-term magmatic process with molten material being intruded into the volcano it may take days to weeks before that shows itself.''
Dr Scott said alpine guides on the Tongariro Crossing took a bad-quality photograph of the volcano early this morning which indicated three vents, about 1km apart, had been activated in the Te Maari Crater area.
"We're dealing with multiple vents. They're not particularly large, looking at the image.''
There had been some further minor seismic activity about 10.30am today, but there had otherwise been no increase in seismicity, he said.
Bunk beds destroyed
Bunk beds inside a Department of Conservation hut on the popular Tongariro Crossing were destroyed by last night's eruption.
The hydrothermal explosion on Mt Tongariro was so powerful it flung boulders, measuring one-metre across, a distance of 1.5km, causing serious damage to Ketetahi Hut.
Mr Peat told media this afternoon that flying boulders had punched holes in the roof and floor of the hut, and destroyed bunk beds inside.
"There was certainly significant damage to the hut so people could have been injured or killed if they'd been inside it.''
Large boulders were dispersed at 10m to 30m intervals in the area surrounding the facility, which is popular with trampers in the summer months.
All four DOC huts around Mt Tongariro will remain closed for safety reasons.
The police-led search and rescue operation on the mountain has now been completed, with no-one found to have been killed or injured as a result of the eruption.
Threat level lowered
Earlier, the Ministry of Civil Defence and Emergency Management said 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".
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."
'Coated in this ash'
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.
