A patient who was recently discharged from Hillmorton told the Herald on Sunday he assaulted a female patient after smoking a legal high at the hospital.
The man - who declined to be named - was given diversion last month at Christchurch District Court.
He said he saw patients abusing staff and other patients after taking synthetic cannabis.
"I had never smoked this stuff before and could not believe the effect it had on me," the man said. "I hit a woman, which I am not proud of. I was so out of it, I was convinced it was a man.
"There is a prison mentality going on among some patients at Hillmorton. There is a lot of peer pressure to try this stuff and also not to nark on anyone who has it."
The man said he had twice witnessed a young female patient crazed on legal highs being restrained by up to 10 staff.
"She took out two nurses and destroyed a wooden picnic table," he said. "Something has to be done about the legal highs before someone is seriously injured, or worse."
Dr Renison confirmed there had been incidents with synthetic drugs but did not believe reintroducing a blanket ban on smoking would stop the trouble.
Other district health boards including Auckland, Wairarapa and Hutt Valley do not allow smoking at psychiatric hospitals.
"We have a committee presently evaluating the temporary measure that allows smoking at Hillmorton," Dr Renison said. "But when smoking tobacco was prohibited, a lot of violence followed, so it is a bit of a double edged-sword.
"Even if smoking was banned, some people would still try to bring legal highs into the hospital, which is strictly against the rules but very difficult to police. These dangerous substances should be made illegal and certainly not be sold at shops near the hospital."
At the City Mission in Auckland, missioner Diane Robertson said problems associated with synthetic drugs had soared in the past year.
"When people with psychiatric problems mix prescription drugs with legal highs they are playing Russian roulette with the outcome," she said.
Last May, three patients at mental health wards controlled by Waitemata District Health Board took High Court action in an attempt to overturn a ban on smoking at closed, intensive-care mental health units.
They lost but the matter is being appealed, said lawyer Richard Francois, who represented the patients.
