A section of the Shotover River, near Queenstown, has been closed as a massive slab of rock threatens to slip into the waterway.
The hanging rock was first noticed on Monday and large gaps are appearing in the rock mass. The volume of rock at risk of falling could be as high as 100,000 cubic metres.
The rock, on the south face of the Shotover River, fell further on Thursday night and was at risk of collapsing into the river and blocking it.
Queenstown Lakes District harbourmaster Marty Black says commercial users of the river have been told, and no-one is in any danger.
An Otago Regional Council consultant geologist has completed an investigation of the subsiding rock from the air and the ground.