Tongariro could erupt again: GNS

The immediate threat of eruption at Mt Tongariro has passed but there remains a chance of another sudden eruption within the next two weeks, GNS Science says.

Twenty-four hours after the eruption from the mountain's Te Maari crater, GNS Science has cancelled its national advisory.

However, it says there remains a "significant probability of a sudden eruption within the next week".

Should a further eruption occur, a new national advisory or warning would be issued, GNS Science said.

Signs of volcanic activity settled overnight but experts continue to monitor the mountain for changes.

Thick grey smoke, gas and ash spewed 4km into the sky from Te Maari vent on the western side of the mountain during the unexpected eruption just before 1.30pm yesterday.

It is the same place where Tongariro erupted in August, for the first time in more than a century.

Yesterday's activity prompted an aviation red alert, which has since been dropped to orange.

Ash in the sky above the mountain also dissipated overnight.

Experts planned aerial observation to check how much gas was in the atmosphere and any other changes, said GNS Science duty volcanologist Nico Fournier.

Scientists have predicted that another eruption of similar size could be expected at any time during the next few weeks, though the activity was not expected to escalate.

"At the moment there are no such signs. It doesn't mean as we saw yesterday or in August that an eruption could not happen; it's quite possible," said Dr Fournier.

"It's a sign of concern for any volcanoes when they don't provide us with any warning."

The Department of Conservation has closed the Tongariro track, which was not expected to be reopened for at least three days.

Dr Fournier advised anyone planning to go to mountains in the area to first check with the Department of Conservation for any safety updates.

Groups of school children and trampers were on the mountain to walk the Tongariro Crossing when the volcano burst into activity, sending many into shock and awe.

Conditions on the mountain were today largely back to what they were before the eruption, with the volcano emitting some steam and gas at the same level it was before the August activity, said Dr Fournier.

"If the roads are open it's pretty much deemed safe and everybody's in touch with us."

Yesterday's activity has also affected flights to and from Taupo, Rotorua or Gisborne airports.

Ten Air New Zealand flights scheduled this morning were cancelled, also causing disruptions at some other regional airports.

The airline would continue to assess the situation, a spokeswoman said.