The controversial youth detention unit at Casuarina prison, Unit 18, will remain open for a further three years while the government builds its $147 million replacement next to Banksia Hill.

Corrective Services Minister Paul Papalia and Corrective Services Commissioner Brad Royce unveiled final designs for the new facility, which will house the state’s most troubled and complex youth detainees, on Friday.

Papalia revealed the tender would be released next week, but it was likely the proposed centre wouldn’t be ready until 2028 and Unit 18 would remain open until the new facility is complete.

The state converted the men’s wing Unit 18 at Casuarina into a youth detention facility in 2022 to remove disruptive young detainees from the Banksia Hill Detention Centre, but it has been plagued with issues since then.

It hit a crisis point when 16-year-old Cleveland Dodd self-harmed in his cell and died a week later – the first child to die in youth detention in WA.

Dodd’s death prompted calls for the facility to close, which the Cook government has agreed to do, but Papalia said it was not possible until the new facility opened.

“Until then, Unit 18 is the best place we have for them, and it’s regrettable that it’s not purpose-designed, but it is the best place we have for them,” he said.

“When it is able to be closed, we will close it, and that will be when we have this purpose-designed facility available.

“Ultimately, it’s a big project. It’s a complex one. It is partly why it’s taken this time to prepare the design, because it is a very complex, very specific build. It’s not something that you do every day of the week.“

The design of the facility comes two years after the Cook government first announced it would build a Unit 18 replacement.

Shadow corrective services minister Adam Hort said the state government should hang its head in shame for the delay in closing Unit 18.

“Western Australians have no faith that this government can deliver anything on time except a new racetrack at Burswood,” he said.

“After the first juvenile death in custody in our state’s history, any government would have acted with urgency to close Unit 18. Instead, today’s announcement makes it crystal clear it’ll be over half a decade before Unit 18 is closed.

“Roger Cook and his government should hang their head in shame for allowing this to define his government’s record - and the record of this state.”

The new facility will have about 30 beds.

The state has been working on designs since November last year, with detainees consulted in the design.

Royce said the detainees offered advice on what items they could break in the new facility.

“They were actually quite informative about what they thought probably would be easier for them to damage,” he said.

“They were given the chance to have a look because it is something that they have a lot of knowledge of, and they’ve been part of the journey, and so have the advocates.”

Papalia described it as a “red-letter day” for Corrective Services in the state.

“If you achieve reduction in recidivism in this cohort, you get disproportionate response and returns for the state,” he said.

Royce warned that construction of the facility in WA’s booming economy would be difficult.

“We are booming as a state, but we’ll be holding on and making sure we get this delivered, as we said we would in the first place,” he said.

The WA government is also looking at the adult prison estate infrastructure and is widely expected to announce a new prison soon.

Population pressure has resulted in judges avoiding prison sentences for some criminals over concerns about conditions in prisons.

Papalia blasted judges who made those decisions.

“What I would say is it is a fact that regardless of their observation about the comfort or otherwise of people inside prisons, it’s the responsibility of the courts to ensure that the community is safe,” he said.

“If that means they are detained in a cell with three people and one of them sleeping on the floor on a mattress. Then, so be it.”