A 26-year-old man who was being treated in North Shore Hospital's mental health unit has disappeared while out on "approved unescorted leave".

Stephen Kapeli left the hospital's in-patient unit late on Wednesday afternoon and did not return at the time he told staff he'd be back by, Waitemata DHB's acting director of mental health Dr Mike Ang said.

Kapeli was wearing shorts, a T-shirt and sports' shoes when he was last seen walking near Lake Pupuke in Takapuna about 4pm.

Staff at the hospital reported his disapperance to police shortly after he failed to return at the agreed time.

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"Waitemata DHB is working with police and encourages anyone who may have seen Stephen to come forward so we can find him and ensure he is safe," said Ang

Detective Sergeant Steve Brewer said police feared for Kapeli's safety because his disappearance was out of character.

"He hasn't used his bank account or cellphone and hasn't been in contact with family which is unusual."

A relative told the Herald Kapeli was expected to return from his walk about 5pm but didn't.

Concerned family members had contacted all his friends but no one had seen or heard from him, the relative said.

"He's very gentle. It is out of character so we are concerned he hasn't contacted us."

The family were worried because Kapeli didn't have his cellphone, wallet or a sweatshirt with him when he left for his walk.

Police want to speak to anyone who may have seen Kapeli in the past three days.

Kapeli is 179cm tall and is of medium to solid build.

"He has an affinity to the water and is known to spend hours at or near North Shore beaches or Lake Pupuke," Brewer said.

Anyone who may have seen Kapeli or has information on his whereabouts can call North Shore CIB on 09 4775 261.