Staff and students from Tamatea Intermediate in Hawke's Bay are reported safe and well after being on the Tongariro Track at the base of the volcano during an eruption this afternoon.
Between 70 and 100 staff and students were about two hours into the track when the eruption occurred.
A school spokesperson said the staff and students were ok and were coming down the mountain.
The school is a regular visitor to the area, with another group having also been on camp last week, when another walk took place to the Ketetahi Springs on the mountain.
Kim Alexander-Turia of DoC at Tongariro said the crossing has been closed at the Mangatepopo and Ketetahi road ends.
She said there were people on the crossing but they would start to return 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.''
DoC was going to let people come down from the crossing ``as they can''.
The crossing would be closed until DoC received word from GNS that it could reopen.
Ms Alexander-Turia said a fly-over later today would confirm everyone had made it off the mountain safely.
A DOC event management team was operating out of the Ruapehu office to co-ordinate the response.
James Perry, who works at the Lake Taupo Hole in One on the 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''.
Further along the lakefront, Melanie Le Prou from Taupo and Carmel Arthur from Perth were also eating lunch when they saw what they thought was a small amount of black smoke above the mountain, but they said they thought nothing of it and did not realise it was an eruption until other people began asking if they'd seen it.
In the Taupo town centre the eruption was clearly visible, with many people noticing it from their office windows.
Tamzin Moore, who works in Taupo, said when her and her colleagues heard on a local radio station that there had been an eruption, they rushed to the windows. With recent talk of Mt Ruapehu being unsettled they expected it to be Ruapehu, but instead saw it was Te Maari Crater on Tongariro that had blown again.
From the Taupo lakefront the ash cloud over Mt Tongariro was visible and as it began to clear, steam could be seen issuing from the vent on the mountain.
State Highway 46, which runs along the base of Mt Tongariro from Rangipo, was closed as a precaution.
WeatherWatch head weather analyst Philip Duncan said that 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.
Air New Zealand said delays and cancellations were not excepted.
The airline was working closely with authorities to keep up to date with ash movements, and would adjust flight routes and altitudes if required.