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Not a second later, a thick grey plume of smoke erupted from 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.