Mount Tongariro has erupted, with smoke pouring out of Te Maari Crater, on the western side of the mountain, shortly before 1.30pm.
A massive plume of ash could be seen coming from the mountain.
GNS Science duty volcanologist Nico Fournier told APNZ the eruption happened about 1.20pm.
"There was one eruption, essentially one explosion, and it was not sustained. It spewed some ash possibly a couple of kilometres up the air. The ash
is now drifting to the east-southeast.''
Programme manager of community relations at DOC in Tongariro Kim Alexander-Turia said the crossing has been closed at the Mangatepopo and Ketetahi
She said there are people on the crossing at the moment, but they were due to start returning from 2pm onwards.
"We haven't sent out a search and rescue because it's not a boulder event - just ash and gas at this time."
She said DOC was going to let people come down from the crossing "as they can".
Ms Alexander-Turia said there was a warden stationed at each hut.
DOC was also running a coordinated incident event at the Ruapehu office.
The crossing will be closed until they received the word from GNS, she said.
Ms Alexander-Turia said a fly-over later today would confirm everyone had made it off the mountain safely.
Adrift NZ, which runs tours of the Tongariro Alpine Crossing, had around 50 people on the mountain today.
Operator Stewart Barclay, who chairs a group of 30 users of Mt Tongariro, was heading to Mt Tongariro to help his guides and their groups off the
"I'm just going to make sure everyone is safe... from my guides perspective and from what I've heard, it seems minor."
Mr Barclay understood the eruption had launched no projectiles into the air.
"I've just talked to one of my guides and he is taking the safe route out. There was a minor amount of panic and everyone is safe now, there were no
Dr Fournier said the eruption was not very loud and was smaller than an earlier eruption in August.
Dr Fournier was a few kilometres away when the eruption happened and said while he was safe, he had "a good view of the explosion''.
"It was remarkable a few minutes ago, now it's dying off,'' he said about 20 minutes after the explosion.
"When it happened you had a column of the ash which spewed in the air vertically, and then essentially the source of that eruption just stopped.''
Dr Fournier said there was "quite a bit of gas'' but it was quietening down and the ash was drifting away.
The next step was to work with the Department of Conservation (DoC) and authorities to make sure people in the area were safe.
"That's going to be the top thing, so right now we are actually with DoC and liaising with the authorities just to make sure that everybody is aware
that there was an eruption, how big it was and where the ash is going.
"Then the authorities will take it from there, if there is any action to be made.
"But at the moment it's probably much less of a big eruption than it was in early August.''
A woman who lives near the mountain said sulphur-smelling ash had been raining down on her home.
Robyn Bennett, who lives 6km from the mountain, said she could see the plume of ash rising into the sky.
"It's sitting under some cloud and that's why it's pushing down onto us, she said.''
"It smells worse than rotten eggs.''
Ms Bennett said the eruption was silent.
She said she spotted a vent today that wasn't there yesterday.
She didn't think she and her husband would need to evacuate, "not unless she starts spewing out a whole lot of red rocks'', but they were waiting to
hear from Civil Defence or the Department of Conservation.
Ms Bennett was not worried about the eruption.
"Why worry? You haven't got time to worry at this stage.''
The ash was drifting south east towards Mt Pihanga, she said.
The Civil Defence said light volcanic ash fall may impact the Waikato, Hawkes Bay, Gisborne and Bay of Plenty areas.
People living in or near the affected the areas should stay indoors and close all windows and doors if ash starts falling. If you are caught outside
seek shelter, or wear a dust mask, handkerchief or cloth to cover your mouth, and protect your eyes with glasses or goggles.
The Civil Defence has a national advisory in effect.
For more on what to do during an ash fall, see here.
GNS Science has increased the aviation colour code from yellow to red following the eruption. The red status means an eruption "is forecast to be
imminent with significant emission of ash into the atmosphere likely".
A police spokesperson said the Desert Road remains open.
Tongariro, in the centre of the North Island, erupted in August for the first time in 115 years, sending ash as far east as Napier.
According to GeoNet, Tongariro is a complex of multiple volcanic cones constructed over a period of 275,000 years.
The mountain's active vents include Te Maari, Emerald, North Crater and Red Crater.
Last week GNS Science increased the likelihood of neighbouring volcano Mt Ruapehu erupting, following increased activity on the mountain.
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