There is no sign of the man who went missing overboard on a Saturday night cruise in Waitemata Harbour in Auckland.

And charter boat operators have told the Herald they don't hold out great hope of finding the man alive with fierce currents and cold water temperatures at this time of year.

A full-scale search operation has been underway after police were called at 9.20pm yesterday with reports of the man going into the water while on a charter boat trip on the harbour.

Police were told the 29-year-old man from Mangere had gone overboard between Kauri Point and Birkenhead, both in the upper Waitemata Harbour.

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The police maritime unit, the police national dive squad and search and rescue volunteers have spent the day searching without success. The search will continue tomorrow.

Charter skipper Marc Taylor, who runs charters out of Westhaven marina, said it was difficult to go over the side of a charter boat without intending to do so.

He said safety standards and intense regulation by Maritime New Zealand had heightened awareness and caution in the charter industry.

"You don't slip over the side when the handrail is up to your waist. You can't control people jumping off the boat. People get dared by their mates - 'I'll give you $50 if you jump over the side'."

Taylor said charter boats had a "zero tolerance" policy that saw charters end for certain behaviour.

"If you jump over the side, that's zero-tolerance. We take your money and go home. Someone jumping over the side could easily turn into a death, especially in the middle of winter."

He said the current in the harbour was strong, reaching between 5-6 knots at the Auckland Harbour Bridge.

"You've got two or three minutes and then someone could be gone."

Another charter operator said there was intense speculation as to which charter boat had lost a client overboard.

The operator also said it was difficult to go over the side of a charter boat, with crew on board to monitor behaviour and limited access to places where clients would be able to fall into the water.

Charter boat operations usually required a group to have a nominated sober person aboard, leaving the skipper and crew able to deal with running the boat and dealing with clients, the operator said.

"This is tragic and everyone is asking who it is. There has to be an investigation and this person's business could do down the toilet.

"Whoever this person is, I feel for them, because someone has lost their life."

A spokesman for Maritime New Zealand - which regulates the charter industry - said it was being briefed by police.