Western Australia’s government is set to face further pressure over plans to move 20 detained children to a maximum-security adult prison.

The boys, aged as young as 14 and mostly Indigenous people, will be relocated from Perth’s Banksia Hill juvenile detention centre to a standalone facility at nearby Casuarina Prison.

Officials say the detainees have been destroying property, escaping from their cells, assaulting staff and harming themselves. They have promised the boys will be kept away from the adult prisoner population in safe and secure units.

But advocates have slammed the decision and called for urgent reform of Banksia Hill after an independent inspector found detainees were having their human rights violated.

Hundreds of people are set to protest in Perth on Thursday against the plan before marching to the WA Department of Justice headquarters.

WA’s custodial services inspector in April revealed there had been multiple occasions at Banksia where several boys spent less than an hour outside their cells.

A group of boys had also made repeated suicide attempts, having formed a suicide pact while isolated within Banksia’s intensive support unit.

About 600 past and present detainees have signed up for a planned class action led by Levitt Robinson Solicitors, alleging they suffered inhumane treatment, were excessively restrained and denied access to education.

The case is before the Australian Human Rights Commission, the first procedural step involved in filing a class action in the Federal Court.

Solicitor Dana Levitt says the decision to transfer detainees to an adult prison shows the extent to which WA’s youth detention strategy has failed.

“This flies in the face of every single international convention to which Australia is a party,” she told AAP.

“Children are only supposed to be detained as a last resort and it’s supposed to be rehabilitative in its focus. They’re never, ever supposed to be put in an adult prison.”

WA’s Department of Justice has said the relocation will allow other detainees at Banksia Hill to return to education and other programs that had been affected by the ongoing disruptions.

About 100 of Banksia Hill’s 260 cells have been seriously affected and more than 30 are unfit for use.

WA Commissioner for Children and Young People Jacqueline McGowan-Jones has expressed alarm over the transfer, saying total separation of the young detainees from adult prisoners at Casuarina is “not logistically possible”.

AAP

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