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The eruption came from Te Maari Crater, on the western side of the mountain, shortly before 1.30pm
A massive plume of ash can be seen coming from the mountain.
GNS Science duty volcanologist Nico Fournier told APNZ the eruption happened about 1.20pm.
"There was one eruption, essentially one explosion, and it was not sustained. It spewed some ash possibly a couple of kilometres up the air. The ash is now drifting to the east-southeast.''
Dr Fournier said the eruption was not very loud and was smaller than an earlier eruption in August.
Dr Fournier was a few kilometres away when the eruption happened and said while he was safe, he had "a good view of the explosion''.
"It was remarkable a few minutes ago, now it's dying off,'' he said about 20 minutes after the explosion.
GNS Science has increased the aviation colour code from yellow to red following the eruption. The red status means an eruption "is forecast to be imminent with significant emission of ash into the atmosphere likely
The initial event 26,500 years ago is the largest eruption and is known as the Oruanui eruption. It ejected an estimated 1170 cubic kilometres of material and caused several hundred square kilometres of surrounding land to collapse and form the caldera. The caldera later filled with water, eventually overflowing to cause a huge outwash flood. It is possible that the Lake Taupo event contributed to starting the Last Glacial Maximum
Civil Defence was warning people to stay indoors if ash was present.
"Light volcanic ash fall can be anticipated downwind of Tongariro and may impact the following areas or regions: Waikato; Hawke's Bay; Gisborne and Bay of Plenty.
"Volcanic ash could be a health hazard, especially if you suffer from breathing difficulty. If outside, seek shelter (e.g. in a car or building)."
It was advising people to close all windows and cover their mouths if caught in the ash. People should stay out of designated restricted zones.
Lake Rotoaira resident Robyn Bennett said there was a big, black ash cloud over her house, which was about a kilometre from the eruption site.
"It's just blew her stack," she said.
She said the air smelled of sulphur.
The biggest worry is Lake Taupo which is in the same Volcanic Chain as Tongariro and Ruapehu. It's one of the worlds largest calderas at over 35km wide.