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Thick gray smoke, gas and ash spewed 4km into the sky from the Te Maari vent on the western side of the mountain, during the unexpected eruption just before 1.30pm.
It is the same place where Tongariro erupted in August, for the first time in more than a century.
Yesterday's activity prompted an aviation red alert, which has since been dropped to orange and the activity alert level remains at two, with no further recorded sign of eruption overnight.
Ash in the sky above the mountain has also dissipated overnight.
But experts were continuing to monitor the situation, including a planned aerial observation to check how much gas was in the atmosphere and any other changes, said GNS Science duty volcanologist Nico Fournier.
They were looking for the same signs they usually do including gas levels and earthquake activity.
Scientists have predicted that another eruption of similar size could be expected at any time during the next few weeks, though the activity was not expected to escalate.
"At the moment there are no such signs. It doesn't mean as we saw yesterday or in August that an eruption could not happen; it's quite possible," said Dr Fournier.
"It's a sign of concern for any volcanoes when they don't provide us with any warning."
The department of Conservation has closed the Tongariro track, which was not expected to be reopened for at least three days.
Dr Fournier advised anyone planning to go to mountains in the area to first check with the Department of Conservation for any safety updates.
Groups of school children and trampers were on the mountain to walk the Tongariro Crossing when the volcano burst into activity, sending many into shock and awe.
Conditions on the mountain were today largely back to what they were before the eruption, with the volcano emitting some steam and gas at the same level it was before the August activity, said Dr Fournier.
"If the roads are open it's pretty much deemed safe and everybody's in touch with us."
• Air New Zealand flights to Rotorua are running again as the ash cloud to the east of the country from Mt Tongariro's eruption yesterday clears.
Flights to Taupo and Gisborne continue to be affected by delays and cancellations, but normal services are expected to resume as the day progresses, said an airline statement.
The airline is working with Civil Aviation Authority, the MetService and other authorities to monitor the situation and make adjustments to routes to ensure aircraft remain clear of any ash.
Passengers are advised to check the Air New Zealand website www.airnewzealand.co.nz for flight arrivals and departures information which will be constantly updated throughout the day.