Thick grey smoke, gas and ash spewed 4km into the sky from Te Maari vent on the western side of the mountain during the unexpected eruption just before 1.30pm yesterday.
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.
Ash in the sky above the mountain also dissipated overnight.
Experts planned aerial observation to check how much gas was in the atmosphere and any other changes, said GNS Science duty volcanologist Nico Fournier.
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.
Ten Air New Zealand flights scheduled this morning were cancelled, also causing disruptions at some other regional airports.
The airline will continue to assess the situation as the morning progresses, said a spokeswoman.
Passengers were advised to check the airline's website for flight details.
SHOCK AND AWE AS VOLCANO BLOWS
As Mt Tongariro started rumbling, children and trampers on the mountain turned to run, fearing the worst.
Not a second later, a thick grey plume of smoke erupted from the Te Maari vent and those closest looked at each other to see what the other was doing.
In one of the school groups on the mountain to walk the Tongariro Crossing, some children cried while others reached for their cameras.
The five-minute eruption sent ash 4km high into the sky without warning and was followed by 15 minutes of volcanic activity, but it was smaller than the August event.
But 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.
Paul Cowan, a teacher from Auckland on holiday, was about 1km to 1.5km away from the 1.25pm eruption. He said there was nowhere to run because smoke was coming out of vents all around them.
"It was fantastic but it was actually a bit scary and everyone started running," he said.
Lynn Donovan, a tourist from Ireland, said that after they heard the rumble they turned around and "all of a sudden" a great tower of thick smoke poured from a crater.
"It was a little nerve-racking and some people started running but a guide from another group calmed us all down and told us what was going on," she said.
During the last eruption, the mountain spewed rocks into the air so they were told to be careful, but otherwise they were safe and there was no need to panic.
Principal John Petrie of Gulf Harbour School, which was doing the crossing with a group of 20 Year 8 students and 10 adults, said they didn't hear anything but some of the children started to notice the billowing cloud of rising ash.
"Initially the kids started getting their cameras out and were quite wowed by it, but as it continued to rise it got quite high and then the apprehension started creeping into all of us," Mr Petrie said. "Some of the kids started crying but others were quite captivated by it."
Last night, continued minor eruptive activity meant Mt Tongariro's Volcanic Alert Level remained at 2.
GNS duty volcanologist Nico Fournier said a northeast wind dragged the ash cloud over Lake Taupo. A light dusting of ash yesterday fell across part of State Highway 46 and towards Turangi.
"We're talking a matter of hours not days for it to fall," Dr Fournier said.
Meanwhile, tourism operators were excited about the eruption.
Adrift NZ tour guide Stewart Barclay, who chairs a group of 30 users of Mt Tongariro, said some school groups had cancelled tours but in the long run the eruption would be "fantastic for business".
He said they had been busier this November than the last because of the eruption in August.
"People just love being near it, they come to gawk at a truly active volcano ..."
Great Lake Taupo spokeswoman, Leola Abraham, said they were trying to focus on the positive impact of the eruption - that it put the region in the international public eye.
And the 8500 people taking part in the Contact Lake Taupo Cycle Challenge this weekend shouldn't change their plans - it will still go ahead.
Event director Kay Brake said they were monitoring the situation closely and there were contingency plans in place.
VOLCANIC ACTIVITY
Yesterday
* 1.25pm.
* Five-minute eruption followed by fifteen minutes of volcanic activity.
* Ash shot 4km into sky.
* No rock blasts or debris flows
* Suspected to be steam-driven.
August 6
* 11.50pm.
* Up to half an hour of eruption then very little volcanic activity.
* Ash shot 7km into the night sky.
* Boulders as large as one metre wide shot high into the air.
* Steam-driven.
* Elevated to Volcanic Alert 2.
* First eruption since 1896 that lasted 11 months.
What next?
* GNS volcanologist Dr Nico Fournier said another eruption of similar size could be expected soon, and there was a possibility of a larger event.
* Ash was being collected and analysed at Massey University to assess health effects. Results were expected in the next few days.
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