Mt Tongariro has calmed down after erupting late on Monday for the first time in over a century, covering parts of the central North Island in ash.
The volcano erupted about 11.50pm on Monday, spewing ash from the Te Maari craters on the northern side of the mountain and prompting a threat warning for the central North Island.
Rocks fell within 1km of the eruption, damaging Ketetahi Hut - one of four trampers' huts on the mountain - but there have been no reports of injuries.
Three people were taken out of Mangatepopo Hut southwest of Mt Tongariro on Tuesday morning, but officials did not think there had been anyone else on the mountain during the night.
Scientists believe the small-scale eruption, which had abated by Tuesday morning, was driven by steam rather than molten rock and say it came without warning.
GNS Science volcanologist Michael Rosenberg told NZ Newswire the eruption would have lasted only a few minutes and was followed by a number of minor earthquakes.
"There'll be a few more minor quakes but for the moment it's pretty quiet but we're just watching it really closely," he said.
"At the moment there is just steam and gas coming through the vents where ash was coming from on Monday night."
The eruption disrupted flights over the central North Island, and aerial news footage shows a fine covering of ash downwind from the volcano.
The National Institute of Water and Atmospheric Research (NIWA) website shows the large ash cloud moving east and blowing offshore hours after the eruption.
GNS Science changed the aviation alert for Mt Tongariro to red overnight, meaning an eruption was forecast with significant emission of ash likely.
However, by midday on Tuesday this had been down-graded to orange - which means the volcano is showing heightened unrest.
A statement from the Department of Conservation says the Tongariro Alpine Crossing, Tongariro Northern Circuit and all four huts on the mountain will remain closed until further notice.
The Tongariro National Park remains open, including both ski fields on Mt Ruapehu which have been unaffected by the eruption.
Police earlier closed some roads in the area as a precaution but these had since been reopened.
Local residents are being warned to check their water supply for ash contamination.
Police were alerted to the eruption shortly before midnight by a member of the public who reported seeing "flame-like explosions" and a cloud of ash coming from "a new hole in the side of the mountain".
Civil Defence said some locals had left the area, but authorities have not ordered an evacuation.
Mt Tongariro last erupted from November 1896 until October 1897.