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Natural causes rather than seismic activity probably triggered the massive landslide in Mt Aspiring National Park.
The landslip, about 150m wide and 150m long, sent at least half a million cubic metres of rock and debris crashing into the John Inglis valley floor burying an alpine lake and blocking a tributary of the Joe River.
Geologist Roydon Thomson, who flew over the scene yesterday, said climate change was probably a major factor with a lot of glacier melt in the area making many of the mountain slopes in the area unstable.
"This was a progressive landslide," he told The Southland Times.
"Landslides in this area are not unusual but rapid falls like this are, which is why we're very interested."
Mr Thomson, who is preparing a report on the slip for Geological Nuclear Science (GNS) at Otago University in Dunedin, said the rumble from the landslide registered 4.3 on the Richter scale.
"But I'm pretty sure this has happened through natural causes and could have been sparked by some glacier action," he said.
"It's definitely not catastrophic but if you were in the area you would need to be very careful."
The slip was spotted by a Queenstown helicopter pilot returning from Milford through the park on January 2.
Heliworks pilot Jason Laing told the newspaper he saw thick dust clouds first and when he got closer he saw the enormous hollowed-out cauldron-shaped hole on the mountain face.
There were rock piles the size of houses on the valley floor, he said.
Wakatipu DOC area manager Greg Lind said trampers in the park should take extreme care.
- NZPA
