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Mr Sinclair (18) has been missing since about 3.30pm on Saturday after he fell into the Clutha River while kneeboarding behind a boat.
Immediate efforts by those on board failed to find him and police were called. More than 100 volunteers then searched unsuccessfully for him.
Sergeant Craig Dinnissen, of Balclutha, said last night the status of the operation had been changed to a recovery operation.
Mr Sinclair's family were ''holding up as best they can'', he said.
The weekend's search had been hampered by poor visibility in the water and the high level of the river,
as a result of spilling from the dams at Clyde and Roxburgh after recent heavy rain in the headwaters.
The search, which focused on the river and its banks downstream of the Waitahuna River mouth, about 4km from Clydevale, included 13 jet-boat teams and nine teams of searchers on foot, and was assisted by a helicopter.
Sgt Dinnissen said yesterday the national police dive squad had also joined the search.
The Sinclair family is well known in the area, having farmed in South Otago for several generations.
When contacted, a family spokesman declined to speak to the Otago Daily Times.
South Otago High School deputy principal John Douglas said Mr Sinclair was a pupil at the school until he left to enter the workforce a year or two ago.
''He's a good kid,'' Mr Douglas said.
Sgt Dinnissen said the community had rallied around the Sinclair family, who wanted to express their ''immense gratitude'' to the volunteers who have been supporting the search. The public support and willingness to help was greatly appreciated, he said.