A young swimmer who got into trouble just outside the flags in the Canterbury surf this afternoon has been rescued by eagle-eyed surf lifesavers.

The 17-year-old man had been enjoying the small waves at Spencer Park Beach just north of Christchurch on a balmy Sunday afternoon.

However, Spencerville Surf Life Saving Club's vice patrol captain Rob Bishop said when the teen started to tire, he soon found himself in danger of drowning.

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Although he had been swimming just outside the lifesaving flags, the on-patrol lifeguards had been keeping an eye on him.

They rushed out in an inflatable rescue boat and plucked him from the sea.

He was checked over by St John paramedics but deemed okay to go home.

"He's had a bit of a fright but he'll be heading off home shortly for the afternoon to put his feet up. He's had enough adventures in the water for one day," Bishop said.

Bishop described the sea conditions as calm, with small waves of less than 1m high.

He said it was a great day for people to enjoy the beach but added the rescue was a timely water safety reminder.

"The ocean can be pretty unforgiving so if you're out of your depth and get a bit tired, it can take over you pretty quickly," he said.

"People need to swim between the flags, try to stay within their depths, keep an eye on signals by lifeguards on the beach, and if you get a bit tired, spend your time on the beach rather than in the water."

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It comes as the Coastguard sends air support to help search for a teenager swept out to sea from an isolated Auckland beach yesterday.

Police have now called off the shoreline search for the 16-year-old after he disappeared from Whatipu while swimming with his family, who tried to save him to no avail.