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A giant rockslide at Aoraki/Mt Cook this afternoon missed a popular trampers' hut by just 200 metres.
A Department of Conservation (DoC) worker said 17 people were using the hut and it was a "miracle" no one was injured.
Chris Braugh from the Helicopter Line spotted the debris during a sightseeing trip and said it was the biggest he had seen.
"It would have been a pretty big roar."
The rocks fell about 900 metres sending clouds of snow high into the air. They came to rest above the Plateau hut, on the eastern side of Mt Cook, by the Hochstetter glacier.
Jim Spencer from DoC told Radio New Zealand the slide was about 3km long and at least 500m wide in places.
The slide, which happened about 2.15pm, looked "ugly", like a "building site with chunks of ice" in it.
Mr Spencer said only some of the 17 people using the hut were outside. He questioned whether those inside would have heard it but said the users were "excited".
DoC was closing the hut as a precaution.
The slide could have been caused by the heavy rain during New Year's week and there was no need for an alert system for slides, Mr Spencer added.