The chance of a whitebaiter being found alive after he disappeared under a large wave is "pretty unlikely".

Police search and rescue co-ordinator Sergeant Ryan O'Rourke said the 21-year-old man was whitebaiting with two friends in rough conditions at the Rakaia River mouth, about 35 kilometres east of Ashburton, when he was swept away about 7pm yesterday.

The Westpac rescue helicopter, with the help of people on the shore, found the man after he had been in the water about 45 minutes, he said.

"In the very cold and difficult conditions, he would have been exhausted ... We've been advised of the timeframes of survival, we think it's pretty unlikely. It's just devastating," O'Rourke said.

He said the Westpac rescue helicopter crew had been hit hard by the rescue attempt, in particular the man who was winched down to try to retrieve him.

The conditions made it difficult to position over the man, who would appear for a few seconds before disappearing under a wave and then pop up about 30 metres away, he said.

O'Rourke said when he finally did get into position over the man he could see a wave coming behind him.

"He realised he wasn't going to make it. He unhooked the rescue tube to throw to him but never got the chance to throw it. It's obviously devastating for him," he said.

An air search for the man continued at "first light" this morning.

"It didn't turn up anything, unfortunately," O'Rourke said.

A land-based search would continue along the north and south of the river this morning. Rakaia Land Search and Rescue (SAR) and Elllesmere LandSAR are assisting in the shoreline search.

- © Fairfax NZ News