Initially the person had been seen bobbing in the rapids, before grabbing hold of a rock.
]The informant left the area to try to get better cellphone reception, but by the time emergency services arrived on the scene there was no sign of anyone in the water and no indication of where they may have gone.
Senior Sergeant Brent Cook, of Greymouth, said emergency services had responded with "full noise" and described the incident as "a very expensive false alarm".
A helicopter was dispatched from Christchurch as the Greymouth-based machine was on another callout, while Kotuku Surf Lifesaving launched boats into the gorge, with spotters up and downstream, and someone volunteered the use of their jetboat.
A large number of on-foot searchers were deployed as well.
After some time it was determined there were no cars in the nearby car park and no sign of anyone.
Senior Sergeant Mark Kirkwood said because the informants - understood to be a group of visitors - had left the area to find a signal, nobody was present to see the distressed individual leave the water.
"It has only been a couple of years since someone drowned in that same spot, and in future we ask that if possible, someone remains to observe to inform searchers," he said.
- By Meg Fulford
