Inmates who should be receiving specialist care by mental health professionals are instead spending weeks at Risdon Prison because the state's forensic mental health facility is full, critics say — claims that have been denied by the Tasmanian Government.

The 35-bed Wilfred Lopes Centre, next to Risdon Prison in the state's south, accommodates prisoners with acute mental illness and is staffed by specialist nurses, psychiatrists and psychologists.

Vice-president of the Law Society of Tasmania, Evan Hughes, said one of his clients had been waiting two weeks to be transferred from Risdon Prison to Wilfred Lopes.

"The only reason that he can't go into that facility is because there just isn't space for him … even though he has all of the issues and conditions that need to be treated by that facility," Mr Hughes said.

The courtyard of the Wilfred Lopes Centre in Risdon Prison ( Supplied: Department of Justice )

Mr Hughes said his client had been assessed by treating psychiatrists and there was no doubt he required treatment for a serious mental illness.

"That must be very difficult for the prison, also for the people at the Wilfred Lopes Centre, and importantly, for my client and his family," he said.

He said Risdon Prison was doing its best to ensure his client was treated well at the prison.

"But the fact is, mental illness needs to be treated like any medical condition and the Lopes is a specially designed facility to help people with mental illness," Mr Hughes said.

"If it's at capacity and can't take people who are vulnerable or have these illnesses and conditions then really we need to have a good, hard look at whether it has the facility that it needs to do its job."

"If [my client] can't be admitted, then I'd be surprised if anyone else can."

Lawyer Greg Barns, from the Prisoners Legal Service, said there were other inmates in the same situation.

"There are a number of people at Risdon Prison who require the acute mental healthcare you get at Wilfred Lopes, who are simply being housed in the prison system because there are no beds available," Mr Barns said.

Nurses' group concern over numbers at centre

The union representing nurses at Wilfred Lopes said the facility's admission area was at capacity.

The Tasmanian secretary of the Australian Nursing and Midwifery Federation, Emily Shepherd, said the 35-bed capacity at Wilfred Lopes and Risdon Prison needed to be increased.

"It's not surprising there are significant delays in access," Ms Shepherd said.

Nurses took industrial action late last year because of significant staff vacancies at Wilfred Lopes Centre, but Ms Shepherd says new staff have since been recruited.

"Obviously staffing is one concern but also actual capacity, in terms of how many patients can be admitted, is another."

A spokesman for the Tasmanian Health Service said the facility was not full.

"The THS is not aware of any issues with capacity at Wilfred Lopes," he said in a statement.

The THS was unable to provide a figure on how many prisoners were at Wilfred Lopes on Saturday.

Risdon 'far from a good situation'

The union representing prison staff says there are ongoing capacity and staffing issues at Risdon Prison as well.

Risdon is operating below capacity, the Government says. ( ABC News: David Fraser )

Tom Lynch with the Community and Public Sector Union said there had been rolling shutdowns at Risdon for months.

"It's far from a good situation for inmates," Mr Lynch said.

"That's meant a lot of inmates have spent a lot of time locked down in their cell. That puts pressure on them, it put pressure on the whole system."

The Department of Justice did not provide a figure on how many prisoners are at Risdon or in which units, saying only that the prison is operating below its designed capacity.

The department said across the state, the Tasmania Prison Service was housing 618 prisoners as of March 29, utilising 94 per cent of capacity.

That is significantly higher than an average of 524 prisoners in 2015-16, and an average of 575 inmates in 2016-17, according to the most recent Productivity Commission report on government services.

Mr Barns said a government obsession with law and order meant police were opposing bail more often.

"And it means we're getting an increase of people in prison who just shouldn't be there," Mr Barns said.

In a statement, Minister Elise Archer said the Government made no apologies for ensuring people who should be in prison, were in prison.