Cleveland Dodd was found unresponsive inside a cell in the youth wing of a high-security adult prison in the early hours of October 12, 2023.
The 16-year-old Indigenous boy was taken to hospital in a critical condition and died a week later, causing outrage and grief in the community.
The Perth inquest into his death had been told Casuarina Prison's Unit 18 was established with little planning in July 2022 as the agency struggled to cope with a small disruptive cohort of young detainees at Banksia Hill Youth Detention Facility.
Cleveland Dodd, 16, died a week after self-harming in his cell at an adult prison's youth wing. (HANDOUT/SUPPLIED)
Counsel assisting Anthony Crocker on Friday read from a series of letters the agency sent to detainees, their families and stakeholders, and the briefing it provided to the corrections minister, seeking approval for Unit 18.
The briefing was an essential document that enabled the government to legally establish the unit.
It also laid out how the "circuit breaker" facility would work and stated it would be suitable for youth detainees and comply with state law.
Former Department of Justice director general Adam Tomison conceded the document he signed contained a series of "grievous lies" that led to the unit being created.
He also agreed the minister should not have been misled and if the government knew the truth it would not have approved Unit 18.
"I accept responsibility as head of the agency," he said.
The briefing stated Unit 18 would have a full suite of services available for detainees, including therapeutic programs, cultural support, and dedicated spaces for health services, education and recreation.
In reality, Unit 18 had few of these and detainees were held in solitary confinement for up to 23 hours per day, often in cells that did not have furniture or running water.
The briefing also said detainees would spend more than nine hours per day outside their cells and there would be up to 120 visitor appointments available five days per week, which was not possible due to ongoing staffing issues.
Dr Tomison agreed it was a "blatant lie" and the people who prepared the document had "lied to the minister".
He told Coroner Philip Urquhart that "it looked pretty good" when he approved the document in July 2022.
The department sent letters about its plans for Unit 18 to detainees, their families and stakeholders.
The letters also made "sloppy" false claims about the services that Unit 18 would have.
Mr Crocker said the conduct was "much more egregious" because the letters were written by senior staff who knew the services could not be provided.
"It is simply wrong, misleading, deceptive to describe it as a full suite of programs," he said.
Dr Tomison conceded it was an "appalling state of affairs" for the agency to write letters containing "untruths".
He also agreed with Mr Crocker that it was "incomprehensible the department would lie to people".
The letter to detainees said "you will have everything you need" in upbeat language, as it spruiked family visits and telephone privileges.
It was far from accurate and in reality, children as young as 14 were handcuffed and transported in a vehicle with blackened windows through a maximum-security prison to a room with no privacy to meet their families or lawyers.
Another letter told detainees' families their loved ones had been "selected" for a new facility with "focused interventions" for young people with "complex needs".
Mr Crocker said this was not true and the cohort sent to Unit 18 were identified by the union representing Banksia Hill's staff.
Mr Urquhart said it was "one thing to paint a rosy picture" for stakeholders, detainees and their guardians, "but quite another to continue with that rosy and false picture ... for a minister".
Asked how he felt about the revelations, Dr Tomison said he was embarrassed and had failed.
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