Scientists have revealed the satellite images of mountain movement that have forced the closure of a climbers' and skiers' hut in Aoraki/Mount Cook National Park.
Murchison Hut was closed last month over fears the mountainside it sits on could suddenly collapse into the glacier about 200m below, killing anyone inside the building.
The urgent decision was made by hut manager the Department of Conservation and owner the New Zealand Alpine Club on advice from experts at Otago University and GNS Science.
The scientists compared French Pleiades satellite images shot in February with an aerial survey's results from 2015 and aerial photographs taken in November 2008.
University surveying lecturer Dr Pascal Sirguey and GNS scientist Dr Simon Cox found the hut, near the head of the Murchison Glacier, had shifted about 9m sideways and 9m downwards over the period of 8 years and nearly four months.
Another set of images in their report, obtained under the Official Information Act, shows numerous cracks in the rock around the hut; these cracks carry on into the nearby snowfield.
Cox told the Herald the cracks spreading into the snow cover was "the smoking gun and seeing it in the snow means that it must be very active".
The two experts met to review the satellite images and surveying data on August 29. The hut was closed the same day.
While climbers and skiers are now told not to stay there, the club has said that the door will not be locked.
The scientists found evidence of a partial collapse of the mountainside below the hut in which the land was lowered by up to 65m.
They estimate that around 500,000cu m of rock has slipped off the slope and may be concealed in the trench between the glacier ice and the mountainside.
"... it ranks amongst some off the significant rockmass failures in the Aoraki-Mt Cook National Park in recent years."
"It appears abundantly clear ... that a large area of bedrock on the side of Mt Cooper is collapsing, likely in response to downwasting [shrinkage] of Murchison Glacier."
The hut area may have stabilised since the February satellite pictures with snowfall and freezing temperatures, but the movement could accelerate with the spring melt.
"Were another collapse to occur from the lower slopes, it is conceivable there could be a retrogressive failure that would generate a cascading collapse that would involve the entire slope from just above the hut all the way down to the glacier - a fall of greater than 200m.
"It is difficult to envisage how any occupant would survive were this to occur."
The risk of instantaneous collapse must be considered by any occupants and by those visiting the site to assess stability, the scientists wrote.
The organisations involved in the hut are planning for a summer site visit to decide its fate.
If the building is removed it will be the fourth hut in the park to go since 2012 - and the fifth since Beetham Hut, near the Tasman Glacier, was smashed by an avalanche in 1996.
Ten-bunk Murchison Hut is popular with commercially-guided and private ski-touring groups although DoC says its usage is light, with around 190 visitors a year.
Gardiner Hut on the western side of Aoraki/Mt Cook was removed in 2015 after being wrecked in a large rock avalanche the year before.
Cox estimated that avalanche off the mountain's South Face, just below the Hillary/South Ridge, involved 900,000cu m of rock.
He said there had been 25 to 30 big rock avalanches in the park in the past 50 years, including the 1991 collapse of the Aoraki/Mt Cook summit.
"I have been working hard to get people to realise just how active the Southern Alps are. I used to think they were stable atua, the gods of the Southern Alps standing there tall and proud.
"It's only in time I realised they change all the time. They are always shaking bits off themselves, falling down or earthquakes are giving them a tickle up."