The finding is contained in a report by the Office of the Inspector of Custodial Services following its most recent inspection of the Banksia Hill detention centre in Perth's southeast.

Inspectors spent five days at the facility last September, when disruptions relating to the coronavirus pandemic had largely ended.

The report points to improvements made at the centre after significant volatility for much of the previous decade, describing the three years prior to the inspection as probably "one of the most settled periods in its history."

At the time there were just 77 detainees at the facility, a number that has since grown to about 110.

But there were concerns about the amount of time detainees spent locked in their cells, including during staff training sessions.

"As a result, young people had an average of only nine to 10 out of cell hours per day throughout 2020," the report published on Tuesday said.

"In our view, this is fundamentally not enough out of cell time."

The issue had been highlighted in previous reports into Banksia Hill, including a review of a 2013 riot which involved more than 70 detainees.

It found that "regular and prolonged lockdowns" had contributed to detainee frustration and to the riot itself.

The latest report urged the state government to invest in a purpose-built crisis care unit for managing detainees who were acutely unwell.

It said the existing observation cells had degraded further and were "not a therapeutic environment for young people in crisis".

Aboriginal people continued to be over-represented, making up almost three-quarters of those detained during the inspection. The youngest detainees were aged 13.

The Department of Justice has expressed full or in-principle support for all 15 of the inspector's latest recommendations.

Commissioner for Corrective Services Mike Reynolds said the department was "working hard to address recent instability".

"Despite a series of recent incidents, Banksia Hill has become a more settled place in recent years under strong and stable leadership," he said in a statement.

"This has allowed staff and service providers to focus on young people's needs and help put them on pathways to better outcomes when they are released."

A report by Social Reinvestment WA last month highlighted that it cost $1339 per day to detain one child at Banksia Hill compared to $93 per day for community supervision.

More than 70 organisations have endorsed the SRWA report which calls for the state to raise the age of criminal responsibility from 10 to 14.

Australian Associated Press