A riot involving about 100 detainees at Western Australia's largest prison is under investigation after some inmates scaled a roof and set fires in their cells.
The riot at Acacia Prison in Wooroloo, on Perth's outer fringes, was brought under control on Sunday night after almost six hours of unrest.
Two officers were treated for smoke inhalation and another was hospitalised with a knee injury after being struck by a projectile.
A prisoner was taken to hospital by ambulance after having a medical episode and also required treatment for smoke inhalation.
The incident came days after after two prisoners and five staff tested positive to COVID-19, prompting restrictions on social visits.
WA's justice department said it was investigating the cause of the disturbance but downplayed suggestions it was related to the virus outbreak.
It said the incident was contained to two blocks within the prison and initially involved about 20 prisoners.
Several climbed onto the roof of their block while others set fire to cells and damaged windows.
Attempts to negotiate with the prisoners were unsuccessful and riot squad officers were deployed.
"At the peak of the incident, a group of prisoners broke away and moved to the industries area of another block within the prison, lighting fires and causing further damage," the Department of Justice said in a statement.
"It's estimated around a hundred prisoners refused to comply with directions from staff at some point during the disturbance.
"No staff were injured and there was no threat to the broader community."
The riot was brought under control about 9.45pm on Sunday.
Acacia, managed by private contractor Serco, currently houses 1462 medium-security prisoners.
WA Prison Officers Union secretary Andy Smith said the facility was "horrendously understaffed".
"Prisoners managed to get their hands on tools, set multiple fires and get within one fence away from freedom," he said.
"The aftermath of this riot will be that Acacia will be even more desperately short staffed today than it was yesterday when prisoners ended up on the roof and setting fires in cells."
Acting corrective services commissioner Mike Reynolds said some staff had been on sick leave.
He told Perth radio 6PR on Monday that short-staffing at the facility was "within the tolerance" of Serco's contract with the state government.
The department said an assessment of the damage would be undertaken and the prisoners involved in the riot were likely to face criminal charges.
