Police and Department of Conservation (DoC) staff have closed the Tongariro Alpine Crossing at Ketetahi and Mangatepopo roads, where the track starts and finishes.
DoC community relations manager Kim Alexander-Turia said about 50 people were thought to be on the crossing at the time of the eruption.
All were thought to be safe and were making their way off the track in their own time.
"The difference between this eruption and the last eruption is there's no volcanic rocks coming out, so we're just letting people quickly and safely come off in their own time, calmly.
"We're just trying to get them off the mountain as soon as possible."
Ms Alexander-Turia said a flyover later today would confirm whether everyone had made it off the mountain safely.
The eruption prompted an aviation alert increase from yellow to red.
Sara Page from GeoNet said the alert was upgraded because the eruption was underway with "significant ash in the atmosphere".
Air New Zealand said there may be delays or cancellations on domestic services to airports east of the mountain.
But the airline said it would adjust flight routes and altitudes if required ensuring aircraft remain clear of any ash.
GNS Science duty volcanologist Nico Fournier told APNZ there was one eruption, "essentially one explosion, and it was not sustained".
Dr Fournier said the eruption was not very loud and was smaller than an earlier eruption in August.
Last week GNS Science increased the likelihood of neighbouring volcano Mt Ruapehu erupting, following increased activity on the mountain.
Dr Fournier said there was "quite a bit of gas" but it was quietening down.
The next step was to work with DoC and authorities to make sure people in the area were safe.
Adrift NZ, which runs tours of the Tongariro Alpine Crossing, had about 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 mountain.
"I'm just going to make sure everyone is safe ... from my guide's perspective and from what I've heard, it seems minor."
Mr Barclay understood the eruption had launched no projectiles into the air.
"There was a minor amount of panic and everyone is safe now, there were no injuries."
Staff and students from Tamatea Intermediate in Hawkes Bay are safe and well after being on the Tongariro Track at the base of the volcano during a volcanic eruption this afternoon.
About 100 staff and students were about two hours into the track when the eruption occurred.
A school spokesman said the staff and students were okay and were coming down the mountain.
The Ministry of Civil Defence and Emergency Management issued a national advisory just under an hour after the eruption.
It said "minor volcanic activity" at Tongariro's Te Maari craters could be hazardous in the immediate vicinity.
Light volcanic ashfall was anticipated downwind of Tongariro and could fall in Waikato, Hawkes Bay, Gisborne and Bay of Plenty.
People living in or near the affected areas were advised to stay indoors and close windows and doors if ash fell. People who were outdoors should seek shelter.
For more on what to do during an ash fall, see here.
Civil Defence said ash could be a health hazard, especially for people suffering breathing difficulties.
People affected by ashfall should wear a dust mask cover their nose and mouth and protect their eyes.
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 didn't think she and her husband would need to leave their home, "not unless she starts spewing out a whole lot of red rocks", but they were waiting to hear from Civil Defence or DoC.
James Perry, who works at the Lake Taupo Hole in One attraction on the resort town's lakefront, had a clear view of the mountains of the central plateau and saw the ash as soon as the eruption occurred.
"It basically went straight up and did the mushroom cloud and then the wind's just spread it from there," he said.
Hole in One owner Tiffany Battell said the smell of sulphur was obvious yesterday and she had wondered then whether it was a precursor to another eruption.
Rhys Harnett of Auckland had stopped on the lakefront to eat his lunch at about 1.30pm when he saw black smoke rising silently from the crater of Mt Tongariro, which he said plumed out above the clouds. He took a photo on his cellphone and rang his home in Auckland to relate what he had seen.
"I had a feeling it was not an eruption but letting off steam."
WeatherWatch head weather analyst Philip Duncan said with only light winds in the region, much of the ash was expected to fall locally.
Lighter ash could travel greater distances and the plume may drift anywhere anywhere from Taupo to Hawkes Bay.
Ash was expected to fall on the Desert Road but was likely to be pushed away from the main air route, to the west of the mountain.
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.
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- nzherald.co.nz / APNZ/ NZ Herald staff
