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The landslip crashed down about noon on Saturday, authorities said, and they warned it posed hazards for trampers, campers and boat enthusiasts.
In a joint statement yesterday, the Department of Conservation (Doc) and the Queenstown Lakes District Council ordered everyone to stay right out of the danger zone.
GNS scientists from Dunedin and Wellington were attempting to assess the landslip and potential hazards by helicopter yesterday afternoon, but were hindered by poor weather conditions, before successfully flying in the evening.
Doc conservation services senior ranger Richard Kennett, of Glenorchy, reported extensive flooding at Dredge Flat, upstream of the slip. Trampers were having to detour 2km through forest to get around flooded parts of the track.
''Doc is strongly advising people to defer planned tramps on the Rees-Dart circuit until the hazard has been fully assessed and more information is available,'' Mr Kennett said.
Harbourmaster Marty Black said boat enthusiasts, kayakers and other recreational users should stay right out of the Dart River until further notice.
While the Dart River's flow has dropped to minimal levels because of the damming effect upstream, there is a risk growing pressure behind the debris dam will breach it, creating a massive surge of water. This presents a constant danger for anyone in the river bed below.
Mr Kennett yesterday afternoon said the water was visibly rising in volume by the hour. By last night the flooding had spread across and up the valley.