There's no way of knowing when Mt Tongariro will next erupt, but another eruption of a similar size could happen at any time over the next few weeks, scientists say.
Tongariro's Te Maari crater erupted for about five minutes on Wednesday afternoon, sending a plume of ash and gas up to 4km into the air.
GNS Science says the eruption - which occurred without any warning - appears to be over, but they haven't ruled out further eruptions over the next few weeks.
There was a similar sized eruption from the same crater in August.
Trampers - including groups of school children - were walking the popular Tongariro Alpine Crossing when the eruption happened about 1.25pm.
About 90 students from Napier's Tamatea Intermediate School were only about 1km away from the Te Maari crater.
"We were right up there next to it. It was just amazing... We saw all these tourists running away from it. We didn't stick around long," teacher Lomi Schaumkel told Fairfax.
Another school group of 20 students and 11 adults from Gulf Harbour School were on their way down the mountain when it sprang to life.
"There was no noise, it was just pluming smoke," principal John Petrie told One News.
"Initially we stayed there looking at it and the kids got out their cameras out and then as it got bigger and bigger we thought `this is interesting'."
There were no reports of injury.
The volcanic alert level remains at two, which signals minor eruptive activity. The aviation colour code has been decreased from red to orange, indicating that a volcanic eruption is underway, but with little or no ash being produced.
August's eruption sent rocks smashing into nearby Ketetahi Hut but Wednesday's burst only contained ash and gas, Conservation Department spokeswoman Kim Turia told NZ Newswire.
Light winds meant the plume went straight up and dissipated, but Civil Defence officials said volcanic ash could fall downwind to the east and people in affected regions should stay indoors.
A spokeswoman for Airways, the organisation responsible for air traffic management, said flights over the central North Island were being diverted away from the volcano.
Air New Zealand cancelled three flights on Wednesday to the main airport nearest the mountain, Taupo, and warned some flights to some airports to the east of Tongariro could be cancelled or delayed.
Last week, GNS warned there was increased activity underneath nearby Mt Ruapehu but scientists have dismissed a link between the two.
In August, the upper Te Maari Crater erupted for the first time in more than a century, sending rocks falling within 1km of the crater.