Victor Boraman, a 21-year-old dairy worker, has not been seen since he was swept away while whitebaiting with two friends on Sunday night.
Three days of searching have failed to find any trace and his family accept he has died.
A death notice published today states that Mr Boraman - a Russian orphan born Victor Sergeyevicvh Vasiliyer who was brought to New Zealand by his new parents Ted and Dixie Boraman as a six-year-old - has been "tragically lost at sea".
A memorial service to celebrate his life will be held at the Papakaio Community Centre in Otago this Friday.
Mr Boraman was originally from Oamaru but moved to Ashburton earlier this year to work on a dairy farm.
According to his mum her son's dog had run into the lagoon.
His friends grabbed it and when they turned around a wave had come in from behind Mr Boraman and knocked him off his feet.
Police said Mr Boraman did everything right.
It's understood he took his waders off so that he could float as attempts were made to rescue him.
Land Search and Rescue co-ordinator Sergeant Ryan O'Rourke said conditions at the time were rough, with strong winds and large waves.
No trace of Mr Boraman has been found since he was seen in the water by a Westpac Rescue Helicopter crew about 8pm on Sunday.
The helicopter was unable to complete a rescue due to the difficult conditions, despite coming agonisingly close.
Shoreline searches for him will continue until Friday if required.
