A series of avalanches on Mount Ruapehu set off alerts to volcanologists when it disturbed the crater lake on Tuesday night.

Up to 50,000 cubic metres of ice and snow became loose on the mountain top, plummeting into the lake and causing a temperature drop of 7 degrees Celsius.

Crack lines reveal where the avalanches occurred from near Mt Ruapehu's crater lake.

Experts suspected there may have been a blockage of geothermal warmth occurring in the lake, caused by the dump.

This meant there was a risk of small "geyser-like" eruptions near the lake, Department of Conservation vulcanology expert Harry Keys says.

Harry, who flew up to the crater lake on Wednesday morning, says the avalanche was "spectacular".

"What's left is a crack, or a crown wall across the top of the avalanche where the avalanche broke away from."

The lake was full and overflowing strongly, Harry says.

"I've been up the crater lake a lot and I don't remember ever seeing so many avalanches in the crater."

Skifields were not affected by the slips.

Thousands of people visit the lake each year.

If anyone had been up the mountain near the lake they probably would have been in danger, says Harry.

"We put a caution on people going too close to the lake, and camping up there, and travelling to the valleys.”

He recommends sticking to the ridges.

Geonet duty vulcanologist Dr Tony Hurst says the data received by Geonet of the avalanche had first appeared as though there had been the result of volcanic activity.

"Our concern was that what had happened was not just a plain avalanche but a volcanic eruption."

However, upon flying up to observe the crater they found "everything was white and clean, so it was purely a mountaineering type avalanche."

The avalanche crashing down was recorded on at least three of our seismometers.

The lake has been monitored continuously since 2009 and the temperature drop, seismicity indicators and lake level had alerted scientists.