An investigation is underway into why a 'frail' man went missing on the day he was released from a rehab facility in Dunedin.

Gordon McLean hasn't been seen since he left Wakari Hospital on Thursday. Search and Rescue, Red Cross, and police dog search teams have been scouring the city for him.

In 2015, a review was conducted after a patient tried to commit suicide by dousing herself in petrol after being released from the same hospital.

But drug and alcohol counsellor, Roger Brooking, said it's difficult for staff, because they don't have the powers to keep patients against their will.

"They can call the police if someone wanders off [but] there's not a lot the police can do about it. They can't lock them up in a police cell and they can't arrest them."

"It's ineffective if you're looking for a lock-up facility where people cannot leave. It just doesn't exist."

"That is probably one of the flaws of the legislation because if people have severe mental health problems, as a result of which they're a risk, they would be placed in a lock up facility. The new legislation for alcohol and drug addicts for compulsory treatment, there is no lock up."

New laws forcing addicts into compulsory rehabilitation will be introduced next year.