A former child detainee at Tasmania's youth detention centre has told an inquiry he was made to perform sexual acts on guards in exchange for his medication.

Warning: Readers may find the details of this story distressing.

The Commission of Inquiry into Child Sexual Abuse in Institutional Settings resumed this week, switching its focus to the Ashley Youth Detention Centre, the only such facility in the state for at-risk youth.

Another witness at the inquiry, criminologist Robert White, said he would want to "raze Ashley to the ground" tomorrow — and that another expert had told him it was "the worst institution" they had ever seen.

Former detainee Warren* told the inquiry he was in and out of the centre about 20 times between the ages of 13 and 18 years.

He was diagnosed with ADHD when he was young, and experienced extreme violence at the hands of his mother.

He told commissioners his mother was trying to have him taken out of her care by inflicting noticeable bruises.

When he first came to Ashley, he tried to keep to his cell, but the guards were regularly required to give him medication for his ADHD.

He told the commissioners that it was during these visits that three of the guards, known only as 'Clyde', 'Reuben' and 'Lionel', began assaulting him.

"The abuse went between oral sex and masturbation, depending on what they could get away with at the time," Warren said.

"I was anally raped over 20 times over my stay at Ashley.

The guards warned him not to report what was going on.

"The workers that were abusing me would threaten me that, if I did say anything, they'd tell the other boys that I was turning them in, so I'd get bashed," he said.

"They would also make threats against my family, saying they would smash up my mum's house and burn it.

"I never told anyone because I was too afraid of what they could do."

'Prison' for kids should be 'razed to the ground'

Professor of Criminology at the University of Tasmania, Robert White, interviewed dozens of staff at Ashley Youth Detention Centre between 2011-12 as part of a review.

He told commissioners he visited the centre with a colleague who had 30 years of experience in prisons and detention centres around the world.

"The moment he walked in the door, he turned to me as an aside and said, 'this is the worst institution I have seen', and it's worse than all the adult institutions that he had visited on all his study tours around the world."

Dr White said back in 2011 his recommendation was that Ashley was not fit for children.

"It's incredible to think that we'd house children and young people in that kind of a place," he said.

"We need to get beyond the euphemism of calling it a detention centre, it really is a prison."

He described a culture where staff were very resistant to change, saw their job as "locking up detainees", and were not able to offer the therapeutic care the children needed.

"We don't have three years of transition; I would get rid of it immediately.

"I think that what we need is a rethink of the philosophy and the mission of juvenile justice."

'Monster' detention centre

Counsel assisting the commission Rachel Ellyard warned the inquiry would learn what happened to Warren was disturbingly commonplace at Ashley.

"It is inherently unsafe for children and has defeated every attempt thus far that has been made to make it safer."

A youth detention centre, as Ms Ellyard put it, is supposed to offer rehabilitation to detainees, who have often experienced significant trauma in their short lives.

However, she said, Ashley operated like a prison, where children as young as 10 were subjected to extreme violence and sexual abuse, were strip-searched and left in isolation by the staff.

She said the behaviour was permitted by a culture of "brutality" towards children, by employees better trained for work as security guards than as custodians of at-risk children.

"A hierarchical and toxic culture in which incidents are not properly reported, and children are threatened and dissuaded from making complaints," Ms Ellyard said.

"That culture can be so pervasive that it corrupts otherwise good people."

Detainee begged to be sent to adult jail instead

Another former detainee, Simon*, first came to Ashley at just 10 years of age.

Like many former youth prisoners, he has spent much of his adult life in prison.

He told the commission that, by the end of his stay at Ashley, he was begging to be sent to Risdon adult jail instead.

"Because of the way I'd been treated there my whole life, you know what I mean? It was disgusting," he said.

He said physical abuse was one thing, but the isolation was worse.

"They chucked me a horse blanket and I slept there for days," he said.

"I was left there for a week or maybe two weeks.

"It was freezing, I'm telling you it was freezing. Felt like it was snowing."

As Ms Ellyard acknowledged, this kind of harrowing evidence is not new.

"The accounts from detainees who were at Ashley in 2000 are distressingly similar to those who were there a year ago," she said.

"As shocking as the evidence is, none of it should be a surprise to the government. None of the evidence should surprise those who worked at Ashley or have been alert to the reports or reviews that have been prepared over the last two decades."

Yet, she said, not enough had been done about it.

Politician asked children commissioner 'to back off'

Mark Morrissey, a former commissioner for children and young people in Tasmania, said he was troubled by what he saw at Ashley.

He told the commission the lack of therapeutic and specialist care at Ashley actually made children more likely to commit future crimes.

He described the isolation of children for long periods of time as a "form of torture".

However, he said, some politicians were resistant to change.

"I received a phone call [from a politician] asking me to understand that any challenges to the current system would affect employment and that it was a very important employer for the Deloraine district," he said.

Mr Morrissey retired early.

Spare underwear due to 'lack of toilet breaks'

The corrections staff at Ashley are known as 'youth workers', however receive nowhere near the same level of training as ordinary youth workers, the commission heard.

"I had, I believe it was three weeks training, part of that was buddy-shifts … I had come from a previous detention centre where I had six weeks training," former worker Sarah Spencer told the commission.

Her colleague, Colleen Ray, said the detention centre was also chronically understaffed, and workers felt unsafe.

"There were always constraints over budget, over staffing, they never did enough recruitment, we couldn't keep enough people," she said.

"So, for a whole period of four years there, there was quite a cohort of staff who were working three and four 12-hour shifts a week."

Things are at times so bad, those on shift can't even go to the toilet, the commission was told.

Isolation punishment defended

In response to a report which revealed children were spending long lengths of time in their rooms — often by choice, according to Ms Spencer — the staff were banned from using isolation as punishment, even for 10-minute intervals.

"Everyone keeps referring to them as children, they're young men, in the gym working out, and they're very hard to manage," she said.

"Without the proper workforce to manage them physically, we had other measures which we are now not allowed to do.

Late last year, the Tasmanian government announced Ashley would close in three years.

While the number of detainees has declined significantly since 2015, there continue to be young people detained there.

"The commission has heard evidence in the lead-up to these hearings about past and recent processes and responses that are of concern," Ms Ellyard said.

"All these have implications for whether the community can presently be satisfied that children who are currently detained in Ashley are safe from the risk of sexual abuse."