A group of 17 teenagers has been relocated from Banksia Hill Detention Centre to an isolated unit at a maximum security prison, despite a backlash from advocates.

The state government announced the plan on July 5 and said the move to Casuarina Prison was due to disruptive behaviour at the youth detention centre.

The boys are aged 14 to 17 years old and were moved to their new facility on Wednesday.

Authorities said the boys would be separated from the adult prisoners in a secure unit.

In a statement, the Department of Justice said the move is temporary and the teenagers will return to Banksia Hill "as soon as practicable".

Director General Adam Tomison said the cohort had significant offending histories and for months had been destroying infrastructure, assaulting staff and harming themselves.

He said the Department of Justice was left with no option than to relocate the detainees to a safer and more secure place.

"We have been managing emergency conditions at Banksia Hill since September last year, leading to a recent escalation in extreme behaviour and numbers of critical incidents, including detainees breaching their sleeping quarters," Dr Tomison said in a statement.

Corrective Services Commissioner Mike Reynolds said the facilities in the newly built unit would provide greater security and safety for the management of the detainees.

"The new setting should allow our staff to have more one-on-one engagement with these young people and help address their complex needs with all the same levels of services that are available at Banksia Hill," he said.

The move caused outrage among advocates, which saw more than more than 75 organisations and community groups sign an open letter to the Premier Mark McGowan, urging him to not send the children to an adult prison.

'Shocking move'

Social Reinvestment WA (SRWA) and the Commissioner for Children and Young People visited the facility on July 12 and said the environment is not suitable for children.

In a statement, Co-Chairperson of RSWA Glenda Kickett, labelled it a "shocking move."

"The unit being used for children is designed for adults, extremely close to adult prisoners and shade cloths will only block views, but not noise.

"This decision needs to be thought through and it needs experts in children's wellbeing guiding the approach," Ms Kickett said.

The coalition of 30 non-profit organisations began its work after a similar move occurred in 2013, when Banksia Hill detainees were sent to Hakea Prison.

SRWA Manager Sophie Stewart said youth justice is in crisis and community-based programs that nurture the young offenders need to be considered.

"Imprisoning children as young as 10 years old is not effective, it is actively harmful, and it is also expensive.

"Sending children to maximum security prison is not our only choice — we can work to fix our broken justice system and keep both kids and communities safe."