Scientists have confirmed a small eruption which may have sent a water plume about 200 metres into the air above Mt Ruapehu after a trip to the mountain top today.
The eruption caused waves about six metres high in the volcanic lake but was a relatively small event which happened reasonably regularly, said Geological and Nuclear Sciences (GNS) coordinator of volcanic surveillance Brad Scott.
He said the scientific team spent some time today on the mountain top.
"There is confirmation as we expected of a very small scale eruption."
Mr Scott said there was no sign of a lahar when volcanic material flows down the mountainside after an eruption.
He said Mt Ruapehu was an active volcano but there had been no similar small scale events for a long time. In the 1980s and 1990s it happened reasonably regularly.
He said the eruption happened last Wednesday night in the middle of a bad storm.
"It would have been a geyser-type eruption. It would have domed up water in the lake and lifted it maybe a couple of hundred of metres high.
"That would have fallen back in the lake and generated waves out across the lake," he said.
Mr Scott said Ruapehu was very active and people had been advised not to go to the summit.