Hundreds of travellers have been left stranded after the eruption of Mt Tongariro on New Zealand's North Island caused road closures and flight cancellations.
The volcano erupted about 11.50pm (2150 AEST) on Monday night - its first eruption for more than a century, spewing ash from the Te Maari craters on the northern side of the mountain and prompting a threat warning for the central North Island.
Several Air New Zealand flights to airports east of Mt Tongariro, including Gisborne, Rotorua, Taupo and Napier, along with Palmerston North, were delayed or cancelled following the eruption, affecting flights to and from Auckland, Wellington and Christchurch.
It's possible that this could be the start of a much longer episode, or alternatively it could be the end. We just don't know.
Australia's airlines, including Qantas, Jetstar and Virgin Australia, confirmed their services to New Zealand are operating as usual today.
Air New Zealand general manager of airline operations and safety Captain David Morgan says the airline is working with the Civil Aviation Authority (CAA), MetService and other authorities to make adjustments to its flight routes and altitudes to ensure its aircraft remain clear of ash and keep travellers flying safely.
The Desert Road (State Highway 1) and SH46 west of Rangipo have reopened after being closed early on Tuesday, while ash has also been reported on SH5 near Te Haroto.
Motel owners in nearby Waiouru, Turangi and Tokaanu told AAP they had not been affected by ash, but there was a sulphur-like smell in the air and dark clouds near Mr Tongariro.
Rangipo Prison, about 20km northeast of the mountain, had not been affected by the eruption and there were no reports of health issues among staff and prisoners, Corrections Department prison services general manager Jeanette Burns said.
There have been no reports of injuries or damage.
Civil Defence said the eruption could pose a threat to Waikato, Hawke's Bay, Gisborne, Manawatu, Wanganui, Bay of Plenty and Taranaki.
GNS Science volcanologist Michael Rosenberg told Radio New Zealand the situation at the mountain was not yet clear.
"For the moment, things are quiet (but that) doesn't necessarily mean that the eruption is over and done with - it could reactivate at any time so we're watching pretty closely.
"It's possible that this could be the start of a much longer episode, or alternatively it could be the end. We just don't know."
Civil Aviation Authority meteorologist Peter Lechner told AAP about 7am local time that the volcano "seems to have stopped erupting at the moment".
If the eruption did cease, the ash cloud should move eastward off the North Island by Tuesday evening.
Truck driver Bryn Rodda told Radio New Zealand the ash on the Desert Road, State Highway 1, was thick with poor visibility before it was closed early on Tuesday.
"I could see this big cloud - it looked like a fist, basically, at an angle across the sky, and about the wrist section of the fist there was an orange ball of flash that I saw."
Civil Defence said some locals had left the area, but authorities have not ordered an evacuation.
GNS Science is monitoring the seismic activity on the mountain, which has been experiencing volcanic earthquakes since July 13.
Mt Tongariro last erupted from November 1896 until October 1897.