A number of people were taken to hospital, including four prison officers, in the latest violent incident involving youths at Perth's Casuarina Prison, a union says.
The incident on Wednesday was the third at Unit 18 in the past two weeks, including one on Sunday, when six guards were allegedly injured after inmates freed themselves from their cells.
While Casuarina is an adult prison, Unit 18 is being used to house a number of youths who had proved difficult to manage at the Banksia Hill Juvenile Detention Centre.
The Community and Public Sector Union said that in Wednesday's incident it was believed three officers were taken to hospital with smoke inhalation, and one had a suspected broken arm.
"In the past two weeks alone, multiple youth custodial officers working at Banksia Hill or Unit 18 have gone to work and left in an ambulance," union assistant secretary Melanie Bray said.
"Others have narrowly escaped serious physical injury.
"The effects of these traumatic incidents continue to compound with every incident that occurs."
She said the continued threat to the safety of officers was a dark shadow over the successful ballot of members, which this week accepted the state government's offer on a new industrial agreement.
The new agreement includes a commitment to baseline measures to begin addressing the serious workplace health and safety concerns held by officers at Banksia Hill.
However, the union said the new regimen did not currently apply to workers or incidents at Unit 18.
"Members have made significant wins for baseline safety measures over the past year, and these incidents continue to highlight how incredibly unsafe these working environments are and how important it is to make safety a priority," Ms Bray said.
"We urge the Department of Justice to continue to work with our members to continue to develop and implement safe solutions.
"Safety must be the first and last consideration at both youth custodial estates"
The department has been contacted for comment in relation to the latest incident.
In Sunday's unrest at the Casuarina jail, two inmates allegedly stole the keys to the accommodation areas and freed 11 others from their cells, before climbing on to the roof and damaging property.
Six guards were injured, debris was thrown at staff, three buildings were damaged and one flooded before the incident ended.
Responding to that incident, Premier Mark McGowan described it as appalling and shocking and said those involved would face charges.