A Massey University volcanologist said there had been a lot of unrest on Mt Tongariro, which erupted last night, since mid-July.
John Procter said while an eruption had not been predicted it was always a possibility.
"It's not a surprise as it's still an active volcano," he said.
" It's the fact that it has occurred [at all] is interesting for us. There has been a lot of unrest and a lot of things. It is always likely to happen though."
This morning, Mr Procter has surveyed the crater from a helicopter.
He said GNS had been noticing changes on the mountain and there had even been reports of a stronger smell of sulphur in the area.
"We saw a steam plume coming from a vent but we couldn't see that vent as it was covered in low cloud. We also saw the extent of where the ash has exploded. It's stretched about 10 kilometres. It's pretty cool to see this in your backyard."
Mr Procter said scientists were now sampling the ash to determine what was happening under the surface.
"It gives us insight into what's happening. If there is new magma then we will find glass shards in the sample… if this is found there could be more eruptions. We will have to wait and see what we find."
He said there was no correlation between Tongariro and the red alert on White Island last week.
Mr Procter has been researching Mount Tongariro and Ruapehu for more than 10 years.
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 a kilometre into the air and across roads, prompting a potential threat warning for central North Island regions.
The "small scale" eruption was a total surprise, GNS volcanologist Michael Rosenberg said.
It threw rocks and spewed ash from the Te Mari craters, near Ketetahi hot springs, on the northern side of the mountain, GNS Science said.
Five to 15 milimetres of ash blanketed nearby properties.
Seismographs this morning were not showing that any more energy was being released, however there was potential for further eruptions. They also showed no activity leading up to or following the overnight hydrothermal, or steam-driven, eruption.
Police urged residents to check water supplies to make sure they were 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 had a predisposition to respiratory issues were at risk, police said.
Three people were evacuated from Mangatepopo Hut in Tongariro National Park and other huts were still being searched by police and Conservation Department staff.
Ski fields were open for business as usual.
There were reports of some schools having masks ready to hand out to children if the situation worsened.
Flights have been affected and some roads closed.
'WE CAN'T REALLY PREDICT WHAT THIS WILL LEAD TO'
Eruptions had occurred on Tongariro intermittently from 1855 through to 1897, and it could not be ruled out that this was the start of a prolonged period of activity in the area, Rosenberg said.
"We have to expect the unexpected. We really can't predict what this will lead to," Rosenberg said.
"This has taken us by surprise. It's gone from some little earthquakes that seemed to be tailing off, then all of a sudden this has gone boom.
"We really didn't expect that there would be an eruption apparently out of nowhere."
Recent volcanic unrest at White Island was unrelated and coincidental to last night's eruption at Tongariro.
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.
He said the sand-like ash had covered his truck, and the sign writing on the trailer was barely visible."It's something I'll probably never see again in my lifetime."
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.
"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.
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.
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.
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, Danya Levy and Jessica Sutton
- © Fairfax NZ News
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