Mt Tongariro could keep erupting for up to nine months, GNS volcanologist Brad Scott says.
Based on the mountain's last cycle of eruptions in the 1890s, it could keep blowing its top intermittently for weeks, or months, the scientist said this morning.
The August eruption was a "vent-clearing blast and naturally more explosive", he says.
Now that there is an open vent, it is easier for eruptions to occur.
GNS volcanologists are meeting this morning to assess the situation.
Nov 21 / UPDATED: Lake Taupo’s annual cycle challenge is under way, with Mt Tongariro’s latest eruption looking "just like another cloud".
GeoNet says the mountain erupted at 1.30pm today – almost four months since an eruption in the same area on Te Maari crater.
Air New Zealand says flights operating to some airports east of Mt Tongariro may be delayed or cancelled as a result of the eruption.
The earlier eruption closed the Tongariro alpine crossing until Labour weekend.
The Lake Taupo cycle challenge began at midday with the enduro riders starting the first of their eight laps of the lake.
Within minutes of the eruption, organiser Hayden Dickason had more marshals out on the course to warn the cyclists of the volcanic activity and to check which direction the wind was blowing.
He says he needs to keep a watch on any ash, because he does not want riders breathing it in.
Mr Dickason told NBR ONLINE he has a contingency plan in place.
The majority of the 8500 entrants will ride just one lap of the lake on Saturday.
“For event day itself, in the unlikely event something big happens we’ve definitely got contingency plans in place to alert people straight away.
"Obviously, we also work with civil defence and police. Should the untoward happen when we need to hold the ride, we’ve got destinations around the lake where people can gather.”
Mr Dickason says he saw the eruption from his office window and because the mountain was covered in cloud at that stage,it just looked like another cloud peeping out the top.
Civil defence has issued a volcanic activity warning and says light volcanic ash fall can be anticipated downwind of Tongariro and may affect Waikato, Hawke’s Bay, Gisborne and Bay of Plenty.
Civil defence says more information will come later in the day from the Department of Conservation and other local authorities with up on the mountain.
Air New Zealand says it is working closely with the Civil Aviation Authority, the MetService and other authorities to keep up to date with ash movement and forecasts.
General manager airline operations and safety, Captain David Morgan, who is also chief pilot, says they are working with the relevant authorities to safely make adjustments to flight routes to ensure aircraft remain clear of any ash.
Passengers are advised to check the Air NZ website for flight arrivals and departures information.
Mt Tongariro has erupted, according to a Geonet report.
The eruption occured in the Te Maari Crater at 1.25am. GNS posted the above image at 1.30pm.
Senior GNS scientist Steve Sherburn said there had been no warning about the five-minute eruption, which was unrelated to concerns about pressure build up at Ruapehu noted by the agency last week.
A large ash cloud is drifting north east.
Robyn Paul from Adrift Guided Outdoor Adventures told NBR ONLINE a company guide was up the track at the time.
However, she says there were no injuries and it appears the track has not been affected.
Ms Paul is about to head up to the track to collect the group of tourists.
Mt Tongariro last erupted in August, for the first time in 116 years, closing the mountain and the Tongariro alpine crossing until Labour weekend.
On October 14 there was a lahar which closed the crossing for a day.
Last week, GNS scientists warned about pressure building under the Crater Lake in a second National Park volcano, Ruapehu.
Conditions were similar to those that preceded Ruapehu eruptions in 2006 and 2007.
ABOVE: Scenes from the August blast.