GNS Science says there has been an eruption at Mt Tongariro in the central North Island with an ash cloud blowing about 2km above the mountain.
The eruption came from Te Maari crater, on the western side of the mountain.
A massive plume of ash could be seen coming from the mountain, and 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.
He 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.
`"When it happened you had a column of the ash which spewed in the air vertically, and then essentially the source of that eruption just stopped.''
The ash was now drifting away, he said.
Light ash fell on Napier following the eruption on August 6 and flights from Napier airport were grounded for a day as a result.
Geonet has changed the Aviation alert code from Yellow to Red, but a spokesperson at Hawke's Bay Airport said flights were still operating at this stage. The red status means an eruption "is forecast to be imminent with significant emission of ash into the atmosphere likely''.
Tongariro, in the centre of the North Island, erupted in August for the first time in 115 years, sending ash as far east as Napier.
According to GeoNet, Tongariro is a complex of multiple volcanic cones constructed over a period of 275,000 years.
The mountain's active vents include Te Maari, Emerald, North Crater and Red Crater.
Last week GNS Science increased the likelihood of neighbouring volcano Mt Ruapehu erupting, following increased activity on the mountain.
A police spokesman said Desert Road remains open.
Adrift NZ, which runs tours of the Tongariro Alpine Crossing, had around 50 people on the mountain today.
Operator Stewart Barclay, who chairs a group of 30 users of Mt Tongariro, was heading to Mt Tongariro to help his guides and their groups off the mountain.
``I'm just going to make sure everyone is safe ... from my guide's perspective and from what I've heard, it seems minor.''
Mr Barclay understood the eruption had launched no projectiles into the air.
"I've just talked to one of my guides and he is taking the safe route out. There was a minor amount of panic and everyone is safe now, there were no injuries.''
Dr Fournier said there was "quite a bit of gas'' but it was quietening down.
The next step was to work with the Department of Conservation (DoC) and authorities to make sure people in the area were safe.
"That's going to be the top thing, so right now we are actually with DoC and liaising with the authorities just to make sure that everybody is aware that there was an eruption, how big it was and where the ash is going.
"Then the authorities will take it from there, if there is any action to be made.
"But at the moment it's probably much less of a big eruption than it was in early August.''