A major landslide - the second this year in Mt Aspiring National Park - has created a new 2km-long lake.
Geologist Geoff Bryant, of Alexandra, said the slip near Lake Wanaka probably occurred between a week and 10 days ago.
The slip has slammed into the North Branchof the Young River, near the head of Lake Wanaka.
"The dam is made up of massive rocks," he said.
The slip started at an altitude of 1500m on Haunted Spur and hundreds of thousands of tonnes of rock plunged 900m to the valley floor, creating a dam 70m high.
So far, the water backing up has created a lake 2km long and 500m wide at its widest point, but the water is still 30m below the crest of the dam and could take several days to reach it.
"The water will settle and eventually flow over the top," said Mr Bryant.
The dam is only 3km above the confluence with the South Branch of the Young River, but downstream of the dam, the riverbed in the north branch is dry. About 1.5km below the dam is a swing bridge and 9km of a popular tramping track on the Gillespie Pass circuit - a four-day tramp that connects the Young and Wilkin Valleys.
Department of Conservation staff have checked the track and are confident there are no walkers in the area, and have erected signs warning of the dam filling up.
"We strongly advise that people stay well away and not travel in the Young at present," said DOC Wanaka area manager Paul Hellebrekers.
Mr Bryant said the size of the rocks in the dam meant there was unlikely to be a catastrophic failure, but the overtopping of the dam would mean a return of waterflows to the dry riverbed.
In January, near Lake Wakatipu on the other side of Mt Aspiring National Park, another massive landslide 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.
It buried an alpine lake and blocked a tributary of the Joe River.
Geologist Roydon Thomson said at the time that 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.
