A landslide has blocked Dart River in Otago, causing flooding upstream and a natural dam that could break at any time.
Officials are warning people to avoid the area after the slip, which happened on the river near Glenorchy, northwest of Queenstown, about midday on Saturday.
It posed hazards for trampers, campers and boaties the Department of Conservation (DOC) and the Queenstown Lakes District Council said on Sunday.
There was extensive flooding at Dredge Flat, upstream of the slip, and trampers have to detour 2km through forest to get around flooded parts of the track, said DOC's senior ranger of conservation services at Glenorchy, Richard Kennett.
The river flow had dropped to minimal levels because of the damming effect upstream, but there was a constant risk pressure behind the slip will force a surge of water through.
This would put anyone in the river below in danger.
Harbourmaster Marty Black said boaties, kayakers and other recreational users should stay out of the Dart River until further notice.
DOC is strongly advising people to defer trips on the Rees Dart circuit until the hazard has been fully assessed and more information is available.
GNS Science experts were assessing the slip and extent of the hazard.
Dr Simon Cox doesn't believe it poses a huge risk to people, but says they should keep in close contact with DOC about what areas are open.
"Some of the Dart camping track will take some time before it's reopened I understand, and they certainly need to check their plans with the Department of Conservation."
Dart River Jet Safaris tourist jet boat trips were suspended while the situation was investigated, a Dart River spokeswoman said.
The landslide happened at Sandy Bluff, about 10km above Chinaman's Bluff.
NZN / RadioLIVE