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A Chinese exchange student died on Saturday night when he went missing during a swim at Christchurch's Sumner beach.

The body of the 19-year-old man, who had only been in new Zealand for a few days, was found on nearby Scarborough Beach early yesterday.

The student was swimming with his host family about 6pm on Saturday when he disappeared, The Press reported.

Police said the youth went back into the water alone and the family lost sight of him.

Searches failed to find him and police were alerted about an hour and a half later.

A large operation was mounted with Coastguard's Sumner Lifeboat, police, volunteers and the Westpac Rescue Helicopter combing the sea and shoreline into the night.

Acting Sergeant Chris Clent, of the Christchurch South police, said the host family were "quite distressed".

The student's family in China were being notified with the help of the Chinese embassy.

It was the second water-related death Christchurch police investigated during the weekend.

A man's body was discovered in a Lyttelton Harbour spillway early on Saturday.

The body was discovered about 5.20am by a security guard who had been alerted by a fishing boat there was something in the water, Detective Sergeant Ross Tarawhiti, of Sydenham police, said.

The body may have been in the water for up to 10 days, he said.

"We're having difficulty confirming his identity, but hopefully in the next couple of days that will be worked on."

The death was not suspicious. Police had an idea of who the man was, but it had not yet been confirmed, Mr Tarawhiti said.

An autopsy had been carried out and the death referred to the coroner.