Climbers warned to steer clear of Mt Ruapehu

Warnings for climbers to stay away from the top of Mt Ruapehu remain in place after seismic activity triggered a lahar alert on the mountain on Wednesday night.

Scientists say it could be a sign of the mountain "waking up" and hope to carry out an aerial survey of the crater lake today after bad weather prevented them seeing if an eruption or lahar had occurred there yesterday.

A lahar alert at the lake was triggered at 10.26pm on Wednesday when an earthquake measuring 2.8 on the Richter scale happened under the crater.

A small lahar was thought to have occurred down the Whangaehu Valley on the mountain's southeast slopes but an inspection of the Whangaehu River from the Desert Rd yesterday showed no sign of lahar activity. "Indications from the monitoring equipment are that any eruption, if there was one, was likely to have been small and any associated lahar too small to be detected off the mountain," Department of Conservation scientist Harry Keys said.

But he said a water-level sensor by the lake was damaged, suggesting a minor eruption or lahar at the lake.

If the weather clears today and scientists are able to fly over the mountain, they will look for traces of discoloured snow. DOC has placed a "high-risk warning" on the crater and is advising climbers not to proceed past the rim.

The Whangaehu, Mangaturuturu and Whakapapaiti valleys further down are considered "medium risk".

GNS Science plans to monitor closely activity on the mountain in the next few days, saying earthquakes directly beneath the crater were sometimes associated with eruptions.

"Ruapehu is quite a difficult volcano to deal with because it often produces earthquakes without eruptions and there can be many of these and then suddenly you get some that have eruptions," GNS Science vulcanologist Steve Sherman said.

"There are not any discernible differences. It seems the waking-up process is done behind a veil.

"What we really have to do is keep a close eye on things, particularly in the next few days, to see whether it's just going to go back to sleep or whether it's going to be the start of another '95 kind of activity."

In 1995, the mountain erupted and sent clouds of ash into the sky and across the central North Island.

Mt Ruapehu skifields were closed yesterday due to high winds rather than the lahar alert.