Canberra Hospital's adult mental health unit has been consistently overflowing since at least 2017, government data shows.
The new government figures also showed Canberra's mental health patients, while making up a small proportion of those admitted to the emergency department, made up 40 per cent of patients staying more than 24 hours in the emergency department.
Canberra Hospital's adult mental health unit was 106 per cent occupied during 2017-18, based on 37 funded beds. The average length of stay during that period was 13.6 days.
The government does not expect the occupancy rate to significantly improve in 2018-19. It's sitting on 104 per cent and an average length of stay of 12.9 days.
The data came in response to questions taken on notice from opposition health spokeswoman Vicki Dunne during the annual report hearings in October.
The response also revealed attacks on staff members in ACT mental health units were on the rise.
It said in 2017-18 there were 98 recorded assaults on staff, the vast majority of them happening at the adult mental health unit at Canberra Hospital.
The government expects those figures will increase for 2018-19 with assaults on staff currently trending upwards.
There were 41 assaults on patients in 2017-18 with that figure trending downwards in 2018-19, the government said.
The response showed there were seven nursing positions vacant in the adult mental health unit and 5.3 vacant positions at Dhulwa - a secure mental health unit - as of November.
Mental health patients also accounted for 150 of the 350 patients in the emergency department who stayed in the department for more than 24 hours in 2017-18.
The government came under pressure last year after nursing staff complained of unsafe working conditions, with the Australian Nursing and Midwifery Federation ACT calling for a greater security presence and a reduction in unnecessary bureaucracy.
Mental Health Minister Shane Rattenbury said mental health services across the country and the world were facing clinical staff shortages.
"At the same time growth in these services has increased. In the ACT, these workforce challenges are felt most keenly across the acute and community settings and are not specific to a particular facility," he said in the response.
Mr Rattenbury said Canberra Hospital offered money incentives - in addition to competitive salaries - to attract qualified psychiatrists.
He said the department had also started a workforce development committee to make a plan to provide a sustainable workforce for the future.
In relation to nurse safety in the mental health wards, Mr Rattenbury said a safety strategy had been finalised and was launched in December.
"There is a dedicated senior nurse in the [adult mental health unit] high dependence unit to assist staff identifying early warning signs of aggression, who also takes the lead in forming a response team to an aggressive incident," Mr Rattenbury said.
He said staff were actively encouraged to contact police if they were assaulted and received training to deal with violent situations.
"As Minister, I support the actions taken within AMHU and Dhulwa," Mr Rattenbury said.
"I am committed to the work under way ... focused on improving work, health and safety for frontline health staff."
