Key Points:

Mt Ruapehu began erupting tonight spilling lahars down two sides of the mountain.

One lahar ran down through the western boundary of the Whakapapa skifield on the mountain.

Highways on both sides of the mountain were closed by police until further notice.

A skifield worker driving a snowgroomer was injured, according to Paul Wheatcroft of the Ruapehu District Council.

There was also a report of a second person injured - a climber who was in a shelter 700 metres from the Crater Lake.

A geologist monitoring the eruption said it had some similarity to the small eruption of June 17, 1996, which also occurred without warning.

Police said activity in the crater lake was noticed by workers on the skifield at 8.30pm.

"Lahars have begun spilling down two sides of the mountain," police said.

Roads on each side of the mountain were closed as emergency services activated a plan developed for the major lahar earlier this year, when one side of the crater lake collapsed.

GNS Science geologists were assessing whether accommodation on the mountain needed to be evacuated, he said. This is the first week of the September school holidays, when many families traditionally visit the Whakapapa and Turoa skifields.

GNS duty volcanologist Craig Miller said one lahar ran down through the western boundary of the Wahakapa skifield, which led to the Whakapapa Stream, but was smaller than the big lahar in March.

It was apparently a "steam-driven" eruption of the type which GNS Science experts warned about after the major lahar on March 18 lowered water levels in the crater lake at Mt Ruapehu.

Mr Miller said there was no warning.

"Looking at our seismic instruments, it was pretty much from nothing to full-on in a minute," he said. "This is probably most similar to the 1969 or 1975 eruptions which occured without warning and when the lake was cold - as it is at the moment".

People near Mt Ruapehu say there was little to see in tonight's eruption.

A man standing outside the Wanganui Ski Club on the Whakapapa ski field, told Radio New Zealand it was "silent as anything" and all that was visible was a faint "unnatural glow" above the mountain.

He said the group were unaware of the eruption until someone's mother called.