Mt Tongariro has erupted this afternoon.
GNS Science confirmed the eruption, at the Te Maari crater, shortly after 1.30pm.
Two bus drivers from Nimon and Sons had reported back to their base that they could see a plume two kilometres high, a spokesman said.
Ruapehu area commander Steve Mastrovich said no-one was injured by the eruption and those on the track were safely making their way down the mountain.
There was no warning leading up to the eruption, which lasted four minutes, Mastrovich said.
"It was fine ash which was ejected and one plume came out then dissipated towards Turangi ... I have been told by somebody at DOC that there were no ballistics ejected," he said.
"There's no further activity from the crater. It's settled down to what it was before."
Tamatea Intermediate teacher Lomi Schaumkel said a group from the school was near the Katetahi hot springs when they saw the beginnings of the eruption.
''We were right up there next to it. It was just amazing. We were probably only a kilometre away from it. We were right next to one of the signs saying we were out of the danger zone.
''We saw all these tourists running away from it. We didn't stick around long,'' he said.
There were 90 students, six parents and four teachers on the group, he said.
Mangatepopo and Ketetahi roads were closed.
Lake Rotoaira resident Robyn Bennett said there was a big, black ash cloud over her house, which was about a kilometre from the eruption site.
"It's just blew her stack," she said.
She said the air smelled of sulphur.
"It's hard to breathe if you go outside, it's pushing out quite heavily."
Kathleen Konui, who lives at Otukou Marae "just below" the mountain said she had heard a sound like a shotgun going off.
"The mountain was all covered with white smoke," she said.
In the hour or so since then the air had cleared, although she could still see some smoke coming from about four holes on the mountain.
Turangi i-SITE Visitor Centre consultant Christine Dally said she was not concerned by the smell of sulphur because there have been many sulphur-smelling days since the August eruption.
"We've had a lot of people ringing us and asking people if it's safe to go up there. We're telling them there's no cause for concern at this stage because it was only a small eruption."
Contact Lake Taupo Cycle Challenge sponsorship and communications director Hayden Dickason said the challenge had a contingency plan in place for Saturday's event as soon as the alert was raised by GNS Science last week.
"[The eruption] looks small at this point, but we're working with council and we've got our safety officer on it. We've got a full contingency plan for the event day in case we need to move everyone off the course."
A northerly wind appeared to be blowing the plume away from Lake Taupo and towards the Desert Rd.
Wellington Airport spokesman Greg Thomas said the situation was being monitored. Flights NZ2517 and 2504 between Wellington and Taupo had been cancelled. Passengers flying in the central region or east coast of the North Island were advised to keep checking if their flights had been cancelled.
GNS Science has updated the alert on Tongariro to 2, meaning there is "minor eruptive activity". It had elevated the aviation colour code to red, however, saying "significant emission of ash'' into the atmosphere was likely.
Massey University is flying two vulcanologists up to study the eruption and already has five students and staff on the ground near the mountain.
More scientists are also on stand-by depending on the size of the eruption.
Massey University vulcanologist Professor Shane Cronin said they will monitor the ash levels and take samples to determine the likelihood of another eruption.
He said the initial suggestion was that the eruption was roughly the same size as the previous Tongariro eruption in August.
This is Mt Tongariro's second eruption this year. The volcano erupted on August 6 for the first time in more than 100 years.
That eruption widened and deepened the Te Maari crater, and reactivated vents which had been covered up in the 116 years since it last erupted in 1896.
Ruapehu has also been active recently but has not erupted. Pressure was building under the volcano and there was an increased likelihood of an eruption, GNS said last week.
The volcanoes are close to each other but scientists did not believe that the activity at both volcanoes was related.
However, they could not exclude the possibility, GNS vulcanologist Nico Fournier said earlier this week.
"We don't have evidence at the moment that the activity of the two volcanoes are related," Fournier said. "But we can't exclude it entirely."
GNS vulcanologists were researching if activity at the two volcanoes was connected and also were looking at the relationship between earthquakes and volcanic activity.
A series of earthquakes rattled the ground beneath Tongariro in the weeks leading up to the August eruption and another series of quakes have shaken the ground beneath Ruapehu in the past few weeks.
HISTORY OF MT TONGARIRO
- 1869: A large eruption (accompanied by an earthquake) formed the upper Te Maari Crater during an explosive eruption. Māori descriptions include a "bright red flame through the smoke that would burst and fall like snow".
- June 1886: An ash eruption occurred from the Upper Te Maari Crater.
- November 1892: Te Maari again belched forth an immense quantity of steam, mud and boulders; the ejected material rose 2,000 - 3,000 feet (600 - 900 m) before rushing down the mountain side.
- January 1893: Gas emissions were reported, along with two explosions, the second of which ejected pumice.
- November 1896: Until October 1897: an eruption occurred from Upper Te Maari Crater, depositing ash in Napier.
- August 6, 2012: The volcano erupts for the first time in more than 100 years. It was a gas-driven eruption and created new vents.
- The Dominion Post
Testing drugs on animals is:Related story: Animal tests 'key' to brain disease cures