St John Ambulance and Granity firefighters were the first emergency personnel on the scene, followed later by surf rescuers, Ms Cargill said.
West Coast search and rescue controller Sergeant Sean Judd said the man had been on a fishing trip with friends when he had decided to go for a swim and got into trouble.
Search and rescue was alerted immediately and a helicopter and two rescue boats were sent out.
Mr Judd said there was nothing particularly different about yesterday's conditions. It was a nice sunny day and the flow level was normal.
"In my opinion in any river mouth, in any river, there's always an element of danger," he said.
He said 20-30 people had helped with the search effort including Buller Surf Rescue, Granity firefighters, St John and locals.
He didn't know whether the man was a strong swimmer, but expected that, as a local, he would have been well aware of the conditions.
At this stage there was no indication that alcohol was involved, Mr Judd said.
Police hoped to name the man this afternoon, after next of kin had been informed.