The Prison Officers Union fears staff may be endangered after two of WA's highest security prisons imposed rolling lockdowns.
The Department of Corrective Services said last night prisoners at Casuarina and Hakea Prisons were in lockdown yesterday.
"Prisoners at both Casuarina and Hakea were confined to their cells for no period longer than four hours," a department spokesman said.
"Restricted regimes are used to ensure the safety of staff and prisoners and maintain the good order of the prison."
Prisoners were still able to exercise, receive visits and attend education programs when not confined to their cells, but other movement was restricted.
_The West Weekend _ understands the lockdowns were triggered after staff shortages became evident at both prisons yesterday morning.
After attempts were made to call in replacement staff, Casuarina was understood to be 34 staff short, while Hakea was down 20 staff.
The DCS spokesman said restricted regimes were put in place for reasons including staff being on sick leave. WA Prison Officers Union secretary John Welch said he feared the situation could worsen at the weekend.
"It's extremely concerning that the two major male maximum security prisons in the metropolitan area are experiencing such enormous shortfalls," Mr Welsh said.
While he said the lockdowns would reduce the immediate threat to staff, he said long periods of confinement created a tense environment for officers to deal with.
"The problem is you can only lock up prisoners for so long," Mr Welch said.
"The prisoners are less likely to be compliant and more likely to be angry, that leads to greater and more numerous incidents of violence, which prison officers are often the target of."
He said the staffing problems were exacerbated by chronic overcrowding.
"The current situation at Hakea and Casuarina means it is critical they hire the required staff immediately," Mr Welch said.
He said the Government needed to recruit more than 200 prison staff to meet current shortfalls across the State.