Indigenous youth justice advocates have called for answers after it was announced the mental health expert appointed to oversee reforms to WA's Banksia Hill Detention Centre would not continue in the role.

Wungening Aboriginal Corporation chief executive Daniel Morrison-Bird told National Indigenous Times that "to let someone of Tim's (Tim Marney) professional experience leave such a vital project at such an important stage is a strange decision".

"To do so without notice or explanation to stakeholders is yet another example of the issues we have raised around transparency and decision making within the Department of Justice," he said.

"I would encourage the Director General, as the leader of this Department, to provide a clear statement setting out the rationale of this decision in order to provide clarity. The absence of clarity is leading to speculation, which will only continue to get in the way of implementing these important reforms.

"I would like to thank Tim Marney for his commitment, genuine collaboration, and professionalism in engaging stakeholders in this important work."

Wungening Aboriginal Corporation is an Aboriginal Community Controlled Organisation which provides culturally secure, confidential and free of charge services to Aboriginal people in the Perth metropolitan area, and also delivers support services through visitor centres at prisons in the metro area.

Noongar human rights law expert Dr Hannah McGlade told National Indigenous Times that advocates are "very concerned" and had "hoped that Tim Marney, who is highly qualified for the role of transforming Banksia Hill would be able to do this job".

"Children are still locked in solitary confinement due to the rolling lock downs and staff shortages. As the Disability Royal Commission recently found, this must stop immediately," she said.

"We've also urged Premier Cook to support an Aboriginal Advisory Committee to guide the culturally appropriate care model but he said it wasn't necessary, which is deeply disappointing. There won't be any reform without a genuine partnership with Aboriginal people, and respect for our knowledge and commitment.

"Tim Marney listened to us and engaged with Aboriginal people, we suggested cultural mentors a be brought into Banksia Hill and he supported this and actioned it. We know he also supported the Aboriginal advisory committee."

Dr McGlade said there is an urgent need for an independent authority to manage Banksia and "ensure the reform actually happens".

"We remain committed to reforming Banksia Hill and urge the Premier to meet us and hear our concerns," she said.

WA's Corrective Services Minister Paul Papalia told National Indigenous Times that the state government thanked Mr Marney "for his work developing the new model of care at Banksia Hill Detention Centre".

"Mr Marney has completed his six month role managing the initial design of the new model and has handed over to the facility's Senior Management Team," he said.

"I recently met with Mr Marney to extend my thanks and gratitude for his considerable contribution.

"A new full-time program director is due to commence with the Department in two weeks."

Mr Marney's six-month contract ended at the end of September.

The Department indicated that a number of initiatives have been progressed under the new model of care and the Youth Justice Subcommittee - comprised of representatives from the Aboriginal Advisory Council of Western Australia, Aboriginal Strategic Advisory Group, Aboriginal Justice Advisory Committee and the Department of Premier and Cabinet's Aboriginal Engagement and Community Policy team - is operating.

The Department also noted that Banksia Hill Detention Centre now has an Aboriginal Services Unit led by an Aboriginal member of the senior management team and supported by Aboriginal Youth Support Officers, Aboriginal Mental Health Workers and other health professionals.