One former detainee, known as Warren, said he was raped and sexually assaulted more than 50 times by guards who withheld his ADHD medication until they abused him.
The commission of inquiry, set up partly in response to abuse allegations against Ashley staff, started seven days of hearings into the centre on Thursday.
Counsel assisting the inquiry, Rachel Ellyard, said there was a culture of brutality and dehumanisation and an over-reliance on strip searches at the centre, which has operated since 1999.
She told the inquiry of a litany of shortcomings including improper use of isolation, "blind spots" in CCTV and a "terrible culture" that could almost be described as anti-child.
"Rather than (it) being about monsters infiltrating. You may find it is Ashley that is the monster," she said during her opening submission.
Ms Ellyard said the centre permitted staff to keep working after they had been accused of abuse, and a culture of nepotism meant workers "backed each other up" when complaints were made.
Warren, who had 21 stints at Ashley from age 13, was regularly forced to perform sex acts on a group of three guards who worked together.
He was held down and raped on multiple occasions and had his arms pinned behind his back and head rammed into a wall by guards who knew he suffered a head injury earlier in life.
"I couldn't do anything to protect myself," he said in a statement read to the inquiry.
Warren was subjected to "degrading" fully naked strip searches and had to bend over in front of guards.
Ms Ellyard said concerns about the centre had been raised in reports to government as far back as 2002.
A 2016 report recommending a shift to two more therapeutic centres wasn't acted upon.
Ms Ellyard said some of the evidence would be "horrific" but none of it should come as a surprise to the government.
"There will be evidence that successive governments have failed to achieve meaningful change," she said.
The detention centre was previously a boys home where some children were sexually abused, and many staff transitioned to the centre when it closed, Ms Ellyard said.
"Some of those still work at Ashley, or did until very recently," she said.
Staff were typically recruited from the local area, Ms Ellyard said, and the centre had been kept open because it was a major regional employer.
The centre's workforce are under-trained and underqualified for dealing with children with complex needs, she said.
The state government in September pledged to close the centre by 2024 and has insisted current detainees are safe.
More than 100 former detainees last week filed legal action against the state government, alleging they were sexually abused or mistreated.
Lifeline 13 11 14
Kids Helpline 1800 55 1800 (for people aged 5 to 25)
13°C