This follows Te Whatu Ora Health New Zealand’s (HNZ) revelation last week that it was considering further reducing Wakari Hospital’s Ward 9a to just eight beds.
The 15-bed unit was operating with a capacity of 10 beds due to staff shortages, including just half the number of nurses required.
The Corrections Association of New Zealand (Canz) has called for better facilities, while the Royal Australian and New Zealand College of Psychiatrists said more forensic psychiatrists needed to be trained.
Corrections deputy chief executive of health Juanita Ryan said prison-based services were not historically designed for people with serious mental illness, but were "doing more than ever" to ensure people in prison had access to the mental health and addiction support they needed.
Many people experienced serious mental illness and some required admission into forensic inpatient services.
"There is a limited number of forensic beds available around the country and we currently have a number of people in our prisons waiting to be admitted into these beds."
Corrections was working with HNZ to provide the best possible care for those in custody, she said.
Funding from Budget 2017 and 2019 meant corrections had improved services, including intervention and support for people in prison at risk of self-harm and suicide.
Canz vice-president Paul Dennehy said the demand for inpatient care had long been an issue, but was probably increasing.
"We have people who are extremely high needs, who would be better placed in an environment such as that, as opposed to inside a prison environment.
"The staff and the medical professionals do their best, but it’s really not the right place for them."
The union believed better on-site mental health facilities within the prison environment were needed.
Waikato's new Waikeria build would have 100 such beds, which was a good start, yet these would be soon filled.
This needed to be replicated throughout the country to ease the pressure.
"It’s a dire situation," he said.
HNZ Southern executive director of mental health, addictions and intellectual disability Toni Gutschlag last week said capacity at Ward 9a would increase again when more staff were recruited.
"We currently have vacancies for registered nurses, enrolled nurses and psychiatrists."
The ward had 12 vacancies for registered nurses, which was more than the 11.1 full-time equivalent it currently employed.
There were 3.4 enrolled nurses and 3.3 positions vacant.
One more senior medical officer position also needed to be filled.
College faculty of forensic psychiatry chairman Dr Justin Barry-Walsh said forensic services had been well resourced when established in the early 1990s.
However, the the prison population had increased, and so had demand.
"At the same time, there hasn’t been growth in the workforce, certainly recently in particular, to meet the growing need."
More funding and the training of more forensic psychiatrists could address the problem, although this would be a long process, he said.
The Public Services Association, the union representing mental health nurses, declined to comment on the situation.
