The union representing doctors at Canberra Hospital has also raised concerns for patient safety inside the adult mental health unit, which has been forced to operate with only one psychiatrist during some shifts in recent months.

Australian Salaried Medical Officers' Federation ACT secretary Stephen Crook said the "crippling shortage" of psychiatrists had followed a period of instability in the ACT's mental health workforce since former Chief Psychiatrist Peter Norrie left the position last year.

Despite raising the crisis with Health Minister Meegan Fitzharris and ACT Health's "senior management", Mr Crook said it appeared the complexities of mental health care provision and the "dangerous effects of excessive workload" on staff were not being recognised.

"Senior management appear to continue to expect more services to be opened and staffed, while current units have inadequate staffing," he said.

An ACT Health spokesman has confirmed nine psychiatrists left ACT Health in the past 12 months, but only six of 13 psychiatrists appointed during that period had actually started.

There were two more psychiatrists who would start work in October and five others would not start until January next year.

He said the ACT's shortage of psychiatrists was part of a larger national shortage and ACT Health had already conducted seven rounds of recruitment advertising through the Australian and New Zealand College of Psychiatrists.

Mental health minister Shane Rattenbury is also expected to raise the issue at the next health COAG meeting of state premiers and the Commonwealth in early August.

There remain eight vacancies across the ACT's mental health, justice health and alcohol and drug services division.

Mr Crook said the staff shortage was most acute in the adult mental health unit at Canberra Hospital, which treats patients with the most severe mental health issues.

He said that unit was "regularly over safe numbers of patients" and despite being designed to be staffed by four psychiatrists, "it has recently been staffed by only one psychiatrist for periods of time".

The health spokesman said the unit had 34 patients as of Thursday and there were currently "2.5 full-time equivalent" consultant psychiatrists, four registrar psychiatrists and two resident medical officers staffing the high-needs unit.

But Mr Crook said registrar psychiatrists caring for "inpatients" were meant to only look after 10 patients at most at any one time, but the staff shortage had regularly pushed that number to up to 16 patients for each registrar in recent months.

The health spokesman also said the directorate has been and continues to organise locum cover to "backfill the shortfall" while permanent staff are recruited.

He said patient and staff safety in the adult mental health unit was the directorate's "number one priority" and was staffed according to a nursing hours per patient day ratio, which the nurses union had agreed to.