Spectacular glacier slide on Ruapehu

A few mini-icebergs float in the crater lake under Paretetaitonga peak (background).

A rare collapse of glacier ice and snow around the banks of Mt Ruapehu's crater lake this week caused parts of the lake to shoot three metres into the air and left mini-icebergs floating in the main body of the lake.

For the past several years scientists have been monitoring the lake as warnings and preparations, including civil defence drills, are made for an expected lahar flow.

Department of Conservation advisory scientist Harry Keys said the level of the lake had now hit 2531m (above sea level), about 5m short of the level where water would "mostly likely" be released.

On Wednesday at 5.30am, about 25,000 cubic metres of snow and ice collapsed in to the lake, enough to fill the equivalent mass of about 30 houses.

"We visited the crater lake to find the ice cliff under Paretetaitonga peak had calved a significant volume of ice, snow and firn (intermediate between snow and ice) into the crater lake," Mr Keys said.

Several waves had gone across the water's surface, muddying banks around the crater's edge. Monitoring equipment had detected significant vibrations at the time.

The level of the lake rose 10cm almost immediately, an event that happened about once a year.

"You seldom see a rise like that. During December we see the most rapid rises. We had 15cm rises each day back when the Manawatu floods hit a few years ago [February 2002]," Mr Keys said.

Although a collapse of ice happened from time to time and this latest one was expected, it was much bigger than usual.

Mr Keys said the sight of mini-icebergs across the lake on Wednesday morning was striking. By 2pm half the many fragments had melted away and the remaining ones would now have also vanished, he said.

The water gets heated by the volcano beneath. The lake's temperature was taken after the collapse. At 13.6C, it was 3C down on last week.

Mr Keys said predictions of a lahar flow were several years old and scientists now believed the event would happen sometime over the next two summers. While a flow could happen in winter, it was "more likely" during the warmer months, he said.

"Right now the crater lake is rising through erosion, melting of ice and snow, and precipitation, but it can lower through evaporation. Each summer the lake has been getting higher."