Canyon Swing general manager Stu Cordelle said one of his staff saw the slip, which had been active for "some time", as it happened yesterday morning and managed to get footage of it.
The company then alerted Go Orange, which runs rafting trips past the slip site.
Go Orange general manager Luke Taylor could not be reached for comment, but it is understood the company suspended rafting trips yesterday.
Mr Cordelle, however, said Canyon Swing operations were not affected.
"It's actually further up the Shotover than we are ... it's from an existing slip.
"That slide's actually been active for some time. From time to time there are some small rockfalls. It's just this time there was a slightly larger rockfall."
Queenstown Lakes District Council harbourmaster Marty black said the area where the slip had occurred was "pretty unstable".
"It has the potential to be quite a big one."
Mr Black said the slip had been "ongoing for some time".
Signs have been erected advising of the closure at the entrance to Moonlight Track, on Moke Lake Rd, and at the top of the Skyline gondola.
The Queenstown Lakes District Council said it would provide an update when it knew more.
Council emergency manager Trevor Andrews said a geologist would be on site today conducting a detailed assessment of the hazard.
