Geonet has confirmed an eruption at Mt Tongariro about 1.30pm.
This image of Te Maari Crater shows smoke and ash billowing from its crater.
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GNS scientist Brad Scott confirmed the eruption but said "it's nowhere near as big as [August's] one".
He said he had not received any reports of damage at this stage, but he was still gathering information.
WHAT YOU NEED TO KNOW
* Airports are all open.Two flights between Wellington and Taupo have been cancelled. Wellington Airport spokesman Greg Thomas said the situation was being monitored.
* Genesis Energy is shutting production at its Tongariro hydro scheme site south of Lake Taupo.
* State highways in the Tongariro area have been closed.
* Tongariro/Rangipo Prison, located near the base of Mt Tongariro, was operating as normal.
* A contingency plan has been put in place for the Lake Taupo Cycle Challenge, to be held at the weekend.
Taupo Cycle Challenge event director Kay Brake said they saw the eruption as it happened from their Horomatangi St premises.
However, they weren't too surprised due to the many warnings they had received from authorities over the past few days.
It did mean they'd step into first gear for their contingency plans for the five extreme cyclists who set out at noon today for their eight-lap ride around Lake Taupo.
Ms Brake said their safety officer and operations director had gone out onto the course to advise the five riders of the eruption.
The riders were continuing on their way, and organisers would now sit tight until further information was received before making any decisions about the races.
They had several contingency measures in place in case there was an eruption, she said.
However, at this stage, the riders and this weekend's around the lake race would continue as planned.
A resident who lives in the shadow of Mt Tongariro has described the "pretty spectacular" eruption that has sent plumes of smoke and ash into the air.
Tony lives about four kilometres away from the mountain and told NewstalkZB Radio he believed about 2000 people would have been walking over the crossing today.
"It's a bit of a worry, isn't it?,'' Tony said.
''I was just talking to a friend in Department of Conservation and they were all getting something underway. Often there could be thousands of people up there on a day during the summer."
Living next door to an active volcano "tests your patience", Tony says.
"It's a bit like living next to the ocean and worrying about a tsunami.''
He says he can see plume rolls of smoke and says he and his whanau had noticed increase smoke in the past several days.
'"We have been sitting here and saying it is going to blow,'' he says.
''I think the scientists call it ground deformation or something. We can see it happening in front of us. It's our view every day.
''This boom here was ash and we are watching it fall over towards the east, towards Potu, the same direction it landed last time.
''This one is not as big as the last eruption, which happened at night. We were lucky enough to see that one, it lit up the skyline almost like a big-screen TV. That time we saw lighting bolts coming out of it and all the drama you see with volcanos.
Tony says he saw a number of scientists dropped off this morning on the mountain at the same spot where this afternoon's eruption came from.''
TIMELINE: TONGARIRO VOLCANO
Mt Tongariro erupted today just before 1.30pm, its second bout of volanic activity this year. Here is a wrap-up of the mountain's history.
August 6, 2012: The volcano erupts for the first time in more than 100 years. It was a gas-driven eruption and created new vents
1869: A large eruption (accompanied by an earthquake) formed the upper Te Maari Crater during an explosive eruption. Māori descriptions include a "bright red flame through the smoke that would burst and fall like snow".
June 1886: An ash eruption occurred from the Upper Te Maari Crater.
November 1892: Te Maari again belched forth an immense quantity of steam, mud and boulders; the ejected material rose 2,000 - 3,000 feet (600 - 900 m) before rushing down the mountain side.
January 1893: Gas emissions were reported, along with two explosions, the second of which ejected pumice.
November 1896: Until October 1897: an eruption occurred from Upper Te Maari Crater, depositing ash in Napier.