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- Mt Tongariro erupted at 11.50pm last night, hurling rocks up to 1km.
- Ash cloud drifting to the east of Tongariro, landing as far as Napier city
- SH1 and SH46 have reopened. Flights to and from Napier cancelled, while other North Island services have been delayed.
- GNS: It was a hydrothermal-driven eruption, rather the magmatic
- It was the first eruption in more than a century
- Turoa and Whakapapa skifields remain open
The aviation colour code around Mt Tongariro has been downgraded from red to orange, but the volcanic alert level remains where it was after the mountain burst into action last night.
Shortly after midday today GeoNet downgraded the aviation colour code, but it has maintained the volcanic alert level at 2.
GeoNet this morning said the plume from the eruption last night was steam-driven, coming from the hydrothermal system rather than from new molten lava rising to the surface.
It shot almost 7000m into the air, "which is not insignificant'', Civil Aviation Authority manager of meteorology Peter Lechner said.
The plume was now sitting over the volcano and to the east towards Hawkes Bay.
"So that block of plume is just quietly drifting away to the east where it will be right off the coast later this evening,'' Mr Lechner said.
GeoNet spokesman John Callan said the seismic activity had died away, as well as the steam plume.
The downgrade of the colour code - which is used to alert aviation operators to activity around a particular volcano - was because there was less ash in the air and the plume was much smaller.
However, it was difficult to predict what would happen next.
"It is too early to predict the next series of events, but we expect heightened activity may continue for several weeks. There are likely to be specific signals of future magma movement beneath the volcano and we continue to monitor the situation through the GeoNet volcano-seismic network of instruments,'' GeoNet said in a statement.
"As with any volcano, an eruption could occur at Tongariro at any time with little or no warning and there is an elevated level of risk, particularly on the northern slopes and valleys of the mountain.''
The aviation colour code uses a traffic light system to alert aviation operators to volcanic activity.
Orange means the volcano is experiencing heightened unrest with increased likelihood of an eruption, while red means eruption is forecast to be imminent.
The volcano alert system rates volcanic activity from zero to five - with zero meaning it was usually dormant and seismic deformation and heat flows were at a low level, while five meant there was a large hazardous eruption in progress.
Level 2 means there is minor eruptive activity.
Helipro pilot Toby Clark took a group of scientists up Mt Tongariro at first light this morning to evaluate the situation.
Bad weather made it difficult to get up to the mountain, but Mr Clark eventually found a space in the weather to the west.
Once above the mountain it was difficult to see because it was covered about 90 per cent by cloud.
"We were able to see the ash plume, or part of the ash plume that was coming up through the cloud, so that was extending up to about 9000 feet.''
They also flew down to check that one of the Department of Conservation huts did not have people in it.
"To be fair, we didn't manage to see the origin of the eruption or the crater.''
The scientists on board were a bit disappointed that they could not see the origin of the eruption, he said.
"But they were pretty happy to see that there was activity there and that it was coming from the area that they thought it was.''
Mr Clark had flown up around the Mt Ruapehu eruption in 1995.
"This, in the GNS words, is nothing like the event of Ruapehu. It was a relatively small event at the moment, but they wouldn't be drawn to the fact that it potentially could go again, or it may not go again, it's really early days.''
The weather for the rest of the day was "not that good'' and there was still a lot of low clouds and drizzle around the mountains, he said.
Ash for one kilometre
Rocks and ash were thrown up to a kilometre from the central North Island mountain after it erupted last night for the first time in more than a century.
The eruption forced the closure of roads and disrupted flights, while nearby residents were advised to stay indoors.
Ash has reached as far as the Napier CBD, as the ash plume drifted to the east of the mountains over the course of the morning.
The volcanic alert level for Mt Tongariro has risen from 1 to 2, while the aviation colour code has been raised to red.
Brent Crowe of the Bay of Plenty police told the press conference that ash and rock was ejected from the volcano in a 1km radius.
The police focus remained on public safety, he said.
Three Department of Conservation huts on Mt Tongariro are being cleared.
All locals residents were urged to remain calm and check water supplies to make sure they were not contaminated.
The eruption was reported to police just before midnight by a member of the public who reported seeing explosions on the northern face of the mountain.
The witness told police the eruption had created "a new hole in the side of the mountain".
Police had received no reports of injury or damage, but the Ministry of Civil Defence & Emergency Management advised people in affected areas to stay indoors.
There have been no mandatory evacuations ordered by authorities, but some residents have chosen to leave the area.
A "potential threat to New Zealand advisory" is in place for Waikato, Hawkes Bay, Gisborne, Manawatu-Wanganui, Bay of Plenty and Taranaki.
Local resident David Bennett said this eruption was "just about as spectacular" as Ruapehu's eruption in 1995. Mr Bennett lives about 6km away from the eruption on the southern shores of Lake Rotoaira
GNS Science volcanologist Brad Scott told reporters it had not detected any increased activity in the 12-14 hours before the eruption.
"There has been no escalation coming into the eruption and we've seen no escalation post the eruption," Mr Scott said. "The eruption gave us no warning ... it just crept up on us."
A flyover of the mountain this morning revealed a steam plume coming from the mountain but there was no indication of volcanic ash this morning.
Scientists were unable to see the immediate impact of the eruption on the mountain because of bad weather and low cloud.
"We were unable to see the impact immediately around the craters so we still can't confirm just which crater the eruptions occurred from,'' he said.
"We've had a small-scale volcanic eruption. It appears to be driven in the hydrothermal rather than the magmatic process, there's been an ash plume, there's been ash-fall down wind.''
Mr Scott said the eruptions are expected to continue for "at least days".
Auckland University volcanologist Phil Shane said the small eruption this morning could signal the start of a pattern of explosions at the central North Island mountain.
He drew a comparison to the Caribbean volcano Montserrat, which rumbled to life in 1995 and has continued to erupt until today.
A more violent eruption in the coming days or a drop off in all volcanic activity was also possible, Associate Professor Shane said.
"We don't know how long it could go. It could be a one off or it could go weeks, months years or even a decade."
Police closed State Highway 1 and State Highway 46 as a precautionary matter overnight but they have since reopened.
New Zealand Herald reporter Jamie Morton said the ash this morning on SH46 at Lake Rotoaira, just north of Tongariro is a "thick, clay-like mud", about half a centimetre thick.
"It's just carpeted everything, all the fields, cars, trees - the whole landscape looks quite murky and grey," he said.
"Roofs in this area, they're all absolutely coated in this ash."
Contractors are currently sweeping the ash from State Highway 46, Mr Morton said.
"There is also quite a noticeable smell ... quite a murky smell. We've had reports in Waiouru that there is a sulphur-like smell in the area. That is not the case here."
Truck driver Bryn Rodda was one of the first to see the eruption.
"As I was coming up from Waiouru .... I saw this beautiful, big cloud and I thought 'gee that looks like a volcanic plume'. Just as I thought that there was a great big orange flash," he told National Radio.
He said he saw a thick cloud of ash develop and fine grey ash started falling.
"(The cloud) looked like a fist, basically, at an angle across the sky. About the wrist section of the fist there was an orange ball of flash I could see.
"It was quite impressive."
Air New Zealand has announced all flights in and out of Hawke's Bay airport are cancelled today due to the ash cloud from the Mt Tongariro eruption.
Further regional flights to and from destinations east of Tongariro could be delayed or cancelled and passengers should check its website for updates.
Captain David Morgan, Air New Zealand general manager airline operations and safety and chief pilot, said the airline is working with the relevant authorities to make adjustments to flight routes to ensure aircraft remain clear of any ash.
"We will not fly through ash and are constantly taking guidance from the CAA and the MetService to ensure we can continue to carry passengers where safe routes and altitudes are available."
Civil Aviation Authority manager of meteorology Peter Lechner said that ash can build up in the turbines of aeroplanes and helicopters, causing engines to stall.
"It can result in significant flight risk."
Federated Farmers said initial reports indicated the eruption had had little impact on farm pasture or stock drinking water.
Four farms had been contacted in different parts of Hawkes Bay, including one on the Taupo-Napier highway, and they were yet to report any ash.
"While we are waiting on reports from our Ruapehu province, GNS predicted ashfall indicates it will largely fall on National Park or forested areas," the group said in a statement.
Skifields on Mount Ruapehu were open this morning, with operators saying the eruption of Mt Tongariro posed no threat to the Whakapapa and Turoa ski areas.
The volcanic vents on Tongariro, Ngauruhoe, and Ruapehu were independent of each other and there was no reason or indication of volcanic activity on Ruapehu, a Mt Ruapehu spokesman said.
"The Te Maari crater where the eruption occurred is at least 20km away from the ski area."
The ash was blowing to the east, away from the skifields.
Nic Peet of the Department of Conservation said facilities around Mt Tongariro were currently closed for public use and he suggested trampers make use of the Whakapapa and Ohakune ends of the mountain.
"Over the next 24 to 36 hours we will make a series of risk assessments about the appropriate time to reopen the facilities," said Mr Peet.
EARTHQUAKES AND ERUPTION
A joint agency incident management centre has been established at the Whakapapa Department of Conservation Visitor Centre.
GNS volcanologist Brad Scott told Radio New Zealand the eruption began from the Te Maari craters at the north end of Mt Tongariro at 11.50pm yesterday.
An earthquake lasting about five minutes accompanied the eruption and residents reportedly heard the explosions.
"When they went outside to have a look they saw the volcano starting to erupt. They saw incandescent blocks - glowing hot blocks - and they saw an eruption column being developed and within about five minutes they were experiencing a light local ash fall."
GNS had been aware of some issues at Mt Tongariro for a few weeks, "but to be honest we didn't see anything in the latest data up until last night that indicated it was ready to erupt", Mr Scott said.
He said there was likely to be further activity.
"There's not showing any escalation - the earthquake activity hasn't increased or anything like that - but we would probably anticipate some more activity now that the craters have broken through."
Mt Tongariro last erupted between 1896 and 1897.
WEATHER AND AREAS AFFECTED
Weatherwatch said where the ash would go depended on the wind.
"We've updated our wind predictions - made more difficult by the fact a large low from the Tasman Sea is expected to cross over Central Plateau. The winds around the centre of the low are very light - which is good news for any further potential ash clouds, as it would decrease the chances of significant drift," the forecaster said in an update.
Civil defence spokesman Vince Cholewa told NewstalkZB ash could reach those living in Waikato, Hawkes Bay, Gisborne, Manawatu-Wanganui, Bay of Plenty and Taranaki.
"The advice to people is to stay indoors, because volcanic ash can obviously be a health hazard, if they're indoors please close windows and doors to try and limit the entry of ash."
Mr Cholewa said not all areas alerted were affected by ash but that situation could change.
"We're working actively with GNS Science who operate the monitoring equipment on the mountains, and with police so all the information from the ground is being gathered, and decisions will be based on that information.
"Evacuations have not been ordered, please listen to the radio for advice from local authorities and police, any evacuations would be issued at that level, and based on the evidence from GNS Science."
Police say the wider community's health is not currently at risk from the eruption of Mt Tongariro.
There was no need for the wider community to remain indoors with closed doors and windows.
"At this time the only risk is minimal and would only be to people in the local vicinity of the eruption who have a predisposition to respiratory issues.
"The only other advice is in relation to water supplies of the rural community in the immediate local area," police said in a statement.
New Zealand's other high profile active volcano, White Island, also had its alert level raised from 1 to 2 on Monday after a small eruption was recorded in its crater lake.
GNS Science said the increased activity on White Island is not related to the Tongariro eruption.
- Herald Online and APNZ