The first eruption in over 100 years at Mt Tongariro may spark months, weeks or even years of volcanic activity - but poses no immediate risk to the community.

Mt Tongariro rumbled into life at 11.50pm last night, sending ash and rock 1 kilometre into the air and across roads, prompting a potential threat warning for central North Island regions.

The eruption was a total surprise, so "we have to expect the unexpected", GNS volcanologist Michael Rosenberg said.

The eruption threw rocks and spewed ash from the Te Mari craters, near Ketetahi hot springs, on the northern side of the mountain, GNS Science said.

Police urged residents to check water supplies to make sure they're not contaminated.

There was no immediate health risk to the community and there was no need to remain inside or keep doors and windows closed, police said.

Only people in the local vicinity of the eruption who have a predisposition to respiratory issues were at risk, police said.

There were reports of some schools having masks ready to hand out to children if the situation worsened.

Civil Defence's Vince Cholewa said local authorities would advise what measures the public should take to keep safe.

Flights have been affected and some roads closed.

There were five reported eruptions from the Te Mari craters between 1855 and 1897 but they had been dormant until now, the GeoNet website said.

WITNESS ACCOUNTS

Truck driver Tama Coker was heading across the Desert Road while the eruption was happening and said the noise was like a train.

"There was a big flash," he said.

"I thought it was lightning and then it started raining sand. It was pretty thick. I heard it rumbling like a train."

Coker said that when he drove through the Desert Road he could not see the white lines on the road.

"I could just see the yellow glare on the mountain. I only had visibility of about 10 to 15 feet in front of me. It was a bit scary.

"It's something I'll probably never see again in my lifetime."

He said the sand-like ash had covered his truck, and the sign writing on the trailer was barely visible.

Local resident David Bennett who lives on the southern shores of Lake Rotoaira, about 6km away from the eruption,  said he heard and saw the mountain erupt just before midnight last night.

He considered himself fortunate no rocks landed on his house last night.

"There were rocks being thrown out. It was like thunder and lightning and fireworks," Bennett said.

There was a lot of ash on State Highway 46 this morning - the roadway linking the Desert Road with the Whakapapa Ski Field.

"It was spectacular. There were rumbling sounds and thunder and lightning coming out from the base of the eruption," Bennett said.

There were no cars in the Tongariro Crossing Carpark when he visited the area this morning, Bennett said.

He said there were about 12 houses on the southern shores of Lake Rotoaira and when the mountain erupted locals made sure all the residents living in the area were safe.

His house is situated about five kilometres away from the roadway leading to the northern end of the Tongariro Crossing.

A few locals did drive to the Hirangi Marae in Turangi but most just stood and watched the spectacular show.

"It's a volcano. If it goes. It will go. We'll all be vapourised. Ruapehu and Ngauruhoe have erupted regularly over the years. Now it is Tongariro's turn."

Bennett's wife Robyn said she had not been able to sleep last night following the spectacular eruption.

"It looked like a huge mushroom cloud. There's a very strong sulphur smell in the air and it was very hard to breathe last night."

Robyn Bennett said she and her husband could still hear the mountain rumbling from their home this morning.

"The ash plume is rolling down the side of the mountain. I feel safe and I am not leaving."

Robyn Bennett said if the mountain did blow "our house will be in the middle of it. The lava flow will come down the valley towards us."

She could see three new vents from her home.

"They each look to be the size of the Ketetahi Springs."

Adventure HQ employee Kerry Wakelin said she took her dogs for a walk about 11.50pm last night.

"I saw flashes and lightning and a big black cloud. I thought it was a big storm," Wakelin said.

She had worked at the Whakapapa Ski Field during the major eruptions on Mt Ruapehu in the mid-1990s.

"When Ruapehu blew back then I had my bags packed and was freaking out. Last night I went to bed and had a good night's sleep," Wakelin said.

She reported feeling small earthquakes in the area during the past two days.

"The latest eruption is like a big old giant who has woken up, farted, rolled over and gone back to sleep," Wakelin said.

A receptionist at the Chateau Tongariro, Maria Dadal, said there was no ash on the Whakapapa Skifield Road this morning and both  the Whakapapa and Turoa skifields would open on schedule this morning.

The manager of the Chateau Tongariro, Tony Abbott, said there were about 100 skier guests at the hotel last night.

"It's business as usual here," Abbott said.

ASSESSING THE SITUATION

Civil Defence earlier said volcanic activity could pose a threat to Waikato, Hawke's Bay, Gisborne, Manawatu-Whanganui, Bay of Plenty and Taranaki.

Teams from GNS would be assessing the volcano by air today and would be gathering samples of ash and rock. If they do not contain magma, then it is indicative that magma has not broken the surface, and may soon do so, Rosenberg said.

Prime Minister John Key was being updated on the eruption by Civil Defence and other Government sources.

He described the volcanic activity as moderate but said Tongariro would continue to be closely monitored.

If the situation required evacuations or an emergency response, Civil Defence would take responsibility of all emergency services.

"At this stage it is very much monitoring," he told TV3's Firstline programme.

Federated Farmers spokesman said the volcanic ash had had minimal impact on farm pasture and stock drinking water.

The Corrections Department said prisoners at the Tongaririo-Rangipo Prison were not in any immediate danger.

NO REPORTS OF INJURIES

An incident management centre had been set up at Whakapapa DOC visitor centre, and local council, police and Department of Conservation were meeting in Taupo this morning.

There had been no further volcanic activity reported since last night.

Civil Defence has not activated the National Crises Management Centre.

Police were hoping to access three huts on the mountain this morning to check whether anyone had stayed there, Ruapehu area commander Steve Mastrovich said.

There had been no reports of injuries or damage because of the eruption and no evacuation notices had been issued.

Rosenberg said some people in the Lake Rotoaira area had self-evacuated following the eruption.

There had been reports of "red hot rocks being thrown out of the crater", several loud explosions and lightning when the eruption occurred and ash had fallen 5cm deep on SH46, he said.

The eruption had been "really unexpected".

"You can measure and monitor but sometimes mother nature will do her own thing."

FLIGHTS AFFECTED

Some flights to and from Gisborne, Rotorua, Taupo, Napier and Palmerston North have been delayed or cancelled due to the eruption.

"We will not fly through ash and are constantly taking guidance ... to ensure we can continue to carry passengers where safe routes and altitudes are available," Air New Zealand said.

About 20 domestic flights in and out of Auckland airport have been cancelled, and 22 have been delayed.

ROADS AFFECTED

The Desert Road section of State Highway 1 had reopened about 8am after earlier being closed. State Highway 46, to the north of the mountain, had reopened at 8.45am.

Motorists were being advised to avoid travel in the area. The road closures would be reassessed at daylight.

Light ash had been reported as falling on SH1 and SH46 and as far as SH5, near Te Haroto and onto Napier city.

- Michelle Cooke, Mike Watson, Marcus Stickley, Zar Lilley, and Danya Levy

- © Fairfax NZ News

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