An average of more than one patient a week absconded from WA mental health facilities between January 2011 and July 31 last year.
In response to questions from shadow mental health minister Ljiljanna Ravlich in State Parliament, Mental Health Minister Helen Morton revealed 86 WA mental health patients absconded in the 19 months.
The incidents were reported to the chief psychiatrist and included voluntary and involuntary patients who left a ward without notifying staff, as well as those who failed to return to a facility at a prearranged time.
Ms Ravlich said the figures showed the mix-up at Graylands Hospital last month, when the wrong person was held and drugged after being mistaken for a patient who absconded two days earlier, was not isolated.
"This is widespread and raises questions about why the system is failing so badly and what the hospitals and the Government are doing about it, if anything," she said.
Ms Ravlich called for a review of procedures to stop patients leaving.
She said there was evidence staff failed to watch mental patients, such as when Kalgoorlie woman Frances Cooper was found dead on October 30 after slipping away from the local hospital on a cigarette break. It is believed a train hit her.
The death was despite her family asking that Mrs Cooper, who had serious mental health issues, be under constant supervision after an earlier escape attempt.
Figures showed there were 15 cases of alleged criminal activity, 16 alleged sexual assaults and 39 alleged cases of aggression or assault involving the patients over the 19 months.
Mental Health Minister Helen Morton said involuntary patients were not criminals to be incarcerated.
"They are receiving involuntary health care in the least restrictive environment as required under the Act," she said.
Involuntary patients were often moved from a secure ward to an open ward and allowed unescorted "leave" for a day, a weekend or longer.
"It is all part of transition back to community living," Mrs Morton said.
Australian Medical Association WA spokesman Paul Skerritt said psychiatrists were encouraged to put patients in open wards because it cost twice as much to lock them away.
He said the alternative to those 86 people absconding would be to lock up more people for longer, which was bad from a civil rights view.