Scientists have declared a ''watch this space scenario'' for Mt Tongariro.
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 total surprise, with the volcano last erupting in 1897, GNS Science said.
Though there had been no escalation in background seismicity today, the mountain could blow its top again depending on what is causing the unrest, GNS scientist Brad Scott said at a press conference in Taupo this afternoon.
''If it is steam driven ... it's unlikely to do much more because once the initial pressure drops occurred you'll just get smaller activity.
''If it is being driven by a longer term magmatic process with molten material being intruded into the volcano it may take days to weeks before that sort of shows itself.''
Ash samples will determine what is driving the activity, he said. ''It's really just a watch this space scenario.''
There was some seismicity detected at 10.30am, which ''may have been some further small scale eruptions or activity, Mr Scott said.
However, there were no visual observations to confirm or deny whether anything happened.
''The only thing that's really come to light that's a little bit exciting is a photograph on a Facebook page taken by some alpine guides on the Tongariro Crossing this morning just on dawn and that shows three vents active in the Te Mari crater area.
''They all appear at this time ... to be new vents. So we're not dealing with a single vent that's been in eruption.''
No one was found injured or dead during a police search of all huts and tracks around Mt Tongariro, Conservation Department Ruapehu-Whanganui area manager Nic Peet.
However, the Ketetahi hut was significantly damaged.
''The track into the hut has got boulders of up to a metre in cross section that have landed on the track and caused impact craters and the hut itself has holes through the roof, the floor and the bunks inside it.''
People could have been injured or killed had they been inside it, Mr Peet said.
It's within 1.5 kilometres of the crater.
Three men who were at the Mangatepopo hut, which was not in the volcanoes firing line, walked out safely this morning.
The Tongariro Alpine Crossing, the Tongariro Northern Circuit and the four huts on the mountain were now closed for public safety reasons, Mr Peet said.
The rest of the national park remains open, including the Whakapapa and Turoa skfields on Mt Ruapehu.
In the next 24 to 72 hours the department will be working with GNS on a risk assessment of the Tongariro National Park facilities and access to tracks.
Inspector Brett Crowe of Taupo police said it will work closely with the experts to determine what assistance they can provide.
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.
CLOUD MOVING OFFSHORE
The ash cloud born from the eruption is moving east and out of New Zealand at speed.
Niwa has created infrared maps showing the movement of the cloud, with the first taken just 39 minutes after the eruption, showing ash bursting into the atmosphere.
Niwa principal scientist Michael Uddstrom said it showed the eruption cloud from the event, which was 25km long and 15km wide.
"The temperature at the top of the cloud (associated with the eruption) is minus 54 degrees centigrade and it's about 12km high at its highest point in the atmosphere."
The second map was taken 113 minutes after the eruption and showed the cloud as it extended toward the east coast of the North Island.
Uddstrom said at 1.45am the highest cloud was just over the coast, south of Wairoa, in the central Hawke’s Bay.
A third map was taken 214 minutes after the eruption showing the cloud which had moved to the south of Mahia.
The cloud has travelled 250km in four hours at an average speed of 18 metres per second, Uddstrom said.
It was rapidly blowing out to the east of New Zealand, Niwa said.
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.
Flights to and from Gisborne, Taupo and Rotorua airports have resumed after earlier flights were cancelled this morning.
However, Hawke's Bay Airport appears to remain closed and flights in and out of Palmerston North continue to be disrupted.
Some flights to and from Gisborne, Rotorua, Taupo and Palmerston North were delayed or cancelled due to the eruption.
All flights in and out of Hawke's Bay Airport have been suspended. Check flight information for Napier airport here: http://www.hawkesbay-airport.co.nz/Flight_Information_14.aspx
- Michelle Cooke, Mike Watson, Danya Levy and Bronwyn Torrie