Mt Tongariro eruption 'a hiccup' - expert
Some regional flights are resuming as scientists advise that the Mt Tongariro volcanic activity is easing.
The volcano rumbled into life at 11.50 last night, sending ash and rock a kilometre into the air, prompting a potential threat warning for central North Island regions.
The "small-scale" eruption was a surprise, with the volcano last erupting in 1897, GNS Science said.
This afternoon, GNS scientists said volcanic activity was easing.
Christchurch International Airport said all flights to and from Napier remained cancelled, but some were resuming to Gisborne, Rotorua, Taupo, and Palmerston North as ash cloud from Mt Tongariro began to clear.
The latest advisory from GNS Science says eruption activity has subsided, and while steam cloud are being observed at the historically active Te Maari craters, there is now no ash being produced from the volcano and there have been no lahars or lava flows.
Waikato civil defence staff have been stood down after the assessment.
A Christchurch Airport spokeswoman said flights between the major centres had been on schedule, although a 5am flight from Melbourne to Auckland had to be diverted to Christchurch and its passengers rerouted on domestic flights.
Air New Zealand chief pilot captain David Morgan said the airline would not fly through ash.
The airline was working closely with the Civil Aviation Authority, MetService and other authorities to keep up to date with ash movement and forecasts.
Civil aviation director of meteorology Peter Lechner said flights between the major centres were less likely to be disrupted because they travelled "well above" the ash plume.
Eruption a 'hiccup'
A Christchurch volcanic expert has called the Mt Tongariro eruption a "hiccup".
Canterbury University lecturer Thomas Wilson said the eruption was relatively small on an international scale.
"It was a smaller rather than a large eruption. I would describe it as a little hiccup for the mountain."
He said further - and larger - eruptions could occur without warning.
"We don't know if this is the beginning of a bigger earthquake sequence or a one-off cough,'' he said.
''If there was going to be a bigger eruption we would expect some warning signals, but it's not guaranteed."
He said last night's eruption was unlikely to trigger any other volcanic activity in the country.
"That's not my area of expertise, but I don't think it's likely."
Wilson said it was hard to predict when volcanoes would erupt.
"The volcanic level of Mt Tongariro was raised a couple of weeks ago because of seismicity and volcanic gas,'' he said.
''That was probably our warning, in hindsight, but it's very hard to forecast when an active volcano will erupt, especially with smaller eruptions such as this one."
The eruption should serve as a warning for New Zealanders, he said.
"Anytime there is a unrest we need to be prepared,'' he said.
''We have a lot of earthquakes in the North Island and I think this has been a good reminder of how active our volcanoes can be."
Eruption 'a total surprise'
GNS scientist Brad Scott said there was strong seismic activity for about 15 to 20 minutes after the eruption, however in the 24 hours prior and hours since, there had not been any significant activity.
There had been no lahars or lava flows, and while it was too early to predict what might happen next, GNS scientists expected heightened activity which could go on for weeks.
It had revised its colour code from red to orange, or from "eruption is forecast to be imminent" to "volcano is exhibiting heightened unrest with increased likelihood of eruption".
The eruption was purely driven by the hydrothermal system, generating steam rather than molten lava, Scott said at a briefing this morning.
Hydrothermal eruptions tended to recur and could escalate into more larger and energetic molten eruptions which spewed out lava and rocks, he said.
GNS was monitoring the activity on the mountain. It had also collected ash samples for assessment.
The agency wasn't yet able to confirm exactly where the eruption came from but it was most likely near the Te Mari craters, near Ketetahi hot springs, on the northern side of the mountain.
The eruption was believed to have lasted only one or two minutes and was followed by a series of small earthquakes.
It generated an ash plume and ash fall, Scott said. Five to 15 milimetres of ash blanketed nearby properties.
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.
Three people were evacuated from Mangatepopo Hut in Tongariro National Park.
Ski fields were open for business as usual.
'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, GNS vulcanologist Michael Rosenberg said.
"We have to expect the unexpected. We really can't predict what this will lead to."
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.
"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.
"It was spectacular. There were rumbling sounds and thunder and lightning coming out from the base of the eruption," Bennett said.
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.
"The latest eruption is like a big old giant who has woken up, farted, rolled over and gone back to sleep," Wakelin said.