On May 6 last year, three detainees allegedly turned on a staff member in a brutal assault that left three employees in hospital.

Four employees have been stood down following an investigation into the incident, which continues, leaving the centre short-staffed.

"Concerns about staff numbers have been known for some time," one source told The Canberra Times.

"Now we have no staff to deliver the programs. A lot of the detainees are sitting in 23-hour lock down."

As staff scrambled to keep on top of things, there was no one around to devote time to the detainees, the source said.

"The young people suffer. There's no one to teach them. No one to kick a football around with them."

Not only were staff concerned about the wellbeing of detainees, they also feared for their own safety, another source said.

"It can be a really dangerous place. But the staff are so resilient and they look out for each other."

It is understood the assault began when, against standard practice, a Bimberi staff member was left to look after detainees in the facility's oval without supervision.

Sources have said staff morale inside the facility has hit rock bottom and there are serious concerns with management's ability to handle the fallout.

"Some days they only have two youth workers," a source said.

"Staff morale is very low at the moment due to shortages.

"Last Thursday, the whole centre needed to be locked down and all visits cancelled for the day due to one young person misbehaving."

The CPSU's ACT regional secretary, Brooke Muscat-Bentley, confirmed Bimberi staff had raised concerns with management.

"We have been monitoring the situation at Bimberi, including through discussions with our members and management," she said.

The ACT's Minister for Disability, Children and Youth, Rachel Stephen-Smith, said the government was focused on the safety of Bimberi detainees.

"My interest is in ensuring the safety of young people in Bimberi and the staff who work there, and in allowing the prescribed processes to be finalised," she said.

"The Community Services Directorate takes seriously its obligations to support staff and to equip them with the skills and abilities to deal with the challenging situations that may sometimes arise in a detention centre."

A spokesman for the Community Services Directorate said there were no issues with staffing levels at Bimberi.

"Staffing levels at Bimberi are appropriate. For instance, at the time of the May 2016 incident there were 13 staff members within the centre supervising four young people," he said.

"There were an equal number of staff members to young people in the vicinity of the incident at that time. The staff were all available to respond at the time of the incident and did so in under a minute from the commencement of the incident."

Bimberi was full of committed staff doing a challenging but rewarding job, the spokesman added.

"Staff employed at Bimberi are committed employees who support some of the most vulnerable young people in our community and are proud to do so.

"When allegations are raised in relation to the behaviour of any employee working in this setting they are taken very seriously and investigated appropriately affording all parties appropriate procedural fairness.

"We acknowledge that such investigations are stressful for staff and can impact upon morale but we are committed to fully investigating allegations and maintaining a high standard in relation to the quality of care provided to young people."

Official Visitor Narelle Hargreaves OAM said that, in her experience, the needs of detainees were well cared for.

"The centre is well managed and the interactions with the young people are constructive and focused on their welfare and wellbeing whilst they reside at the centre.

"I wish to assure the ACT community that the 'voices' of the young people are heard and listened to by the management at Bimberi."

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This article was originally published at 3pm on Friday March 17. It has been updated to include comments from the Community Services Directorate.