Latest: Tongariro eruption has 'minimal impact'

Federated Farmers says there there is no threat to pasture or livestock drinking water stemming from Mt Tongariro's eruption last night

Mt Tongariro's on heightened alert after its first eruption in 100 years.

The eruption's described as small and was centred at the Te Maari craters.

People living near Lake Rotoaira, at the southern end of Lake Taupo, reported the eruption to police about 11:50pm last night.

Do you have photos or video of the eruption? Email michaeloliver@radionetwork.co.nz

Federated Farmers says the eruption has had minimal impact on farms in the area.

A spokesperson says there’s relief the eruption is not as bad as it could be, and ash fall has been limited to forests near the central plateau.

The group will send an advisory out to farmers later this morning with details on how to cope with any further eruptions.

Horizons Regional Council is advising farmers living near the volcano or downwind to disconnect water tanks that may have been contaminated by ash fall.

GNS Duty Volcanologist Michael Rosenberg eye witnesses reported seeing red hot rocks coming out of the crater lakes on the north side of Tongariro and ash falls up to five centimetres deep on both the Desert Road and State Highway 46.

Tongariro's activity alert has been raised from one to two on a scale with the highest number five -  that means a small eruption is underway.

However the Civil Aviation alert has been raised to red, telling pilots there is ash being thrown into the air.

Michael Rosenberg says they raised the alert level on Tongariro about two weeks ago, after a series of small earthquakes under the mountain.

But he says those quakes had become smaller and fewer so they thought activity had declined.

"Unfortunately it's not possible to predict what these eruptions are going to do and certainly in this case with the eruptions being so unexpected all we can do is monitor very closely."

Mr Rosenberg says it was a fairly small eruption but they're watching it very closely to see what happens next.

He says there's no connection between the eruption on Tongariro and the increased activity at White Island.

Ruapehu Area police commander Steve Mastrovich says search and rescue volunteers have flown onto the mountain to check if anyone's caught in tramping huts.

"We haven't encountered any difficulties with anybody, haven't encountered anybody in fact so that seems to be ok, there's nobody in any particular danger on the mountain," he told Newstalk ZB's Mike Hosking.

Mr Mastrovich says the section of State Highway One between Rangipo and Waiouru, which was closed by ash after the eruption, has reopened.

State Highway 46 across the northern face of the mountain, west of Rangipo is still closed due to the possible danger to the public.

Mr Mastrovich says the wind is carrying the ash east towards Hawke's Bay.

No evacuations but warning over ash

While Civil Defence has not ordered any evacuations, a number of families have voluntarily left their homes.

Taupo District Council emergency manager Phil Parker says until they know more, there's no need for residents to evacuate.

"There's nothing that's causing us any concern at this stage and the way the weather forecast looks I don't think it will so it's really just a monitoring situation that we're in."

Mr Parker says they believe the mountain has quietened down for now.

The biggest problem today is expected to be the ash which is five centimetres deep in some areas near the mountain - and has fallen as far east as Napier.

Civil defence spokesman Vince Cholewa says ash could reach those living in Waikato, Hawke's 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 says at this stage not all areas alerted are 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."

Newstalk ZB's Adam Walker is at Whakapapa and says plans are being put in place.

He says making things more difficult for officials is the thick blanket of fog in the area.

Flights to the east of the North Island are likely to be affected following the eruption.

Flights affected by eruption

Newstalk ZB understands all flights in and out of Gisborne this morning have been cancelled due to the ash cloud.

Air New Zealand says flights operating in and out of Rotorua, Taupo, Napier and Palmerston North may also be delayed or cancelled.

Chief Pilot Captain David Morgan says Air New Zealand's will make adjustments to flight routes to make sure aircraft stay clear of any ash.

Airways spokeswoman Philippa Sellens says planes can avoid the ash.

"There is absolutely no threat to airplanes flying, we will take them on a flight path around or above the ash cloud, we will certainly have a very wide safety margin."

The Civil Aviation Authority's confident the ash will clear later this evening, and any disruptions to flights should be confined to this morning.

Civil Aviation Authority manager of meteorology Peter Lechner says the ash plume is leading off to the east and south east.

"Flight operations to the west of the plume should remain unaffected, however operations to the eastern half of the North Island will have some difficulty at this stage."

Mr Lechner says a frontal system is on its way which should disperse the cloud, but that depends on whether there are further eruptions.

Photo: NZ Herald