WA's custodial services inspector has delivered the finding after visiting Perth's Banksia Hill last December, describing conditions there as severely understaffed and "inhumane".
The unscheduled visit, which followed an increase in self-harm and staff assaults, resulted in WA's Department of Justice being issued with a rare show-cause notice.
In a harrowing report released on Tuesday, inspector Eamon Ryan said there had been 24 suicide attempts at Banksia Hill between January and November last year.
Most involved several boys who had formed a suicide pact while being held under observation in the facility's "intensive support unit" (ISU).
Mr Ryan found there had been several days in November where four detainees spent less than an hour outside of their cells.
"We concluded that their human rights were being breached on those occasions," he said.
The United Nations' Mandela rules, which are not legally enforced in WA, require that detainees spend at least two hours out of their cell in a 24-hour period.
Detainees who had pre-existing trauma and cognitive impairments were being denied meaningful interaction, resulting in more instances of self-harm, Mr Ryan said.
He warned that while the department may not have unlawfully confined detainees, it could still be vulnerable to a legal challenge.
Hundreds of former detainees have signed up for a class action alleging mistreatment at the facility.
"(Banksia Hill) is not fit for purpose as a youth detention centre," Mr Ryan said.
"It looks like, and in many respects runs like, an adult prison, even to the point where there are adult prison officers stationed there to assist in maintaining order and security."
One 16-year-old ISU detainee, known as Daniel, attempted suicide five times in one month and had to be resuscitated in his cell.
He had been quieter and calmer since being placed in the girl's precinct which had better access to staff, education and recreation, Mr Ryan said.
"It is fortunate that Banksia Hill has never had a death in custody or suicide," he said.
"However, the likelihood of this remaining the case is diminishing as the conditions in the ISU aggravate the mental health of detainees and incidents of attempted suicide continue to increase."
The McGowan government will invest $25 million into Banksia Hill in next month's budget.
It will fund a new crisis care unit and improvements to the ISU, including new recreation areas and observation cells.
The department says about 40 staff have recently started working at Banksia Hill, undergoing training on trauma and cognitive impairments.
It said there had been fewer unscheduled lockdowns and detainees were spending more time out of their cells, including in the ISU.
Advocate Gerry Georgatos said the funding was paltry given the state's $5.6 billion budget surplus.
"These children are our most critically vulnerable," he said.
"They are layered with trauma, nearly all of it unaddressed."
Lifeline 13 11 14
Kids Helpline 1800 55 1800 (for people aged 5 to 25)
5°C