Detainees are locked in “shocking and horrific” conditions that are further damaging already vulnerable children, a press conference for a class action against the West Australian government over the treatment of up to 500 inmates was told.

Social justice campaigner Gerry Georgatos said the detained children were denied access to rehabilitation or adequate medical care.

He said he had witnessed “abominable and abhorrent behaviour” towards inmates and a statement by the state government that there was access to therapeutic services was untrue.

“It’s actually an outright lie and outrageous by the government,” he told reporters on Friday.

“It does not exist.”

Lawyer Stewart Levitt said many of the children were often victims of intergenerational trauma, who needed support and education so they could become contributing members of society.

“It’s not accepted in Australia, we just want to bash and hurt and punish and pretend that we can just consign these children to dungeons effectively, or treat them as vermin or pests,” he said.

“That is a totally inhumane approach.”

Former President of the WA Children’s Court Denis Reynolds detailed the case of a boy who appeared in the court in November.

He said the 15-year-old suffered fetal alcohol spectrum disorders, post traumatic stress disorder and multiple severe learning disorders.

A pediatrician recommended he be given a support program while incarcerated but he was instead locked in a cell for over 20 hours per day for 42 days straight due to alleged staff shortages, Mr Reynolds said.

He said he witnessed ‘extraordinary’ treatment of inmates by riot police in the centre, with some forced to walk backwards down stairways.

The teen was also allegedly unlawfully transferred to the adult facility, Casuarina Prison, for 51 days and kept in solitary confinement.

“He attempted to commit suicide,” Mr Reynolds said.

“The premier and the minister are saying these are bad children behaving badly ignoring, deliberately, any reference to the unlawful treatment.”

Mr Reynolds alleged Premier Mark McGowan was misleading the public and the issue the government needed to tackle was the unlawful and inhumane treatment of children.

Former Australian of the Year Fiona Stanley said the WA government was ignoring evidence about the children that showed the current punitive approach at Banksia Hill would not work.

“They should understand that children are being born with brain damage (and they) should not be locked up in these circumstances,” she said.

WA Greens MP Brad Petitt said some children were being sent to Casuarina from Banksia Hill “almost immediately” and the government’s statement that only those who were difficult to manage were transferred was untrue.

Dr Petitt, who recently toured the facility, said the government had failed to provide requested information and transparency was lacking.

He said he witnessed “extraordinary” treatment of inmates by riot police in the centre, with some forced to walk backwards down stairways.

Mr Levitt said parents of inmates were being denied access to information about their children, along with experts seeking to inspect conditions in Banksia Hill.

He accused the WA government of withholding internal audit reports and public service whistleblowers were “petrified” about having their careers ruined for speaking up.

The allegations come about week after an affidavit filed to the Federal Court revealed a teenage girl with autism was confined to a Banksia Hill cell for up to 23 hours a day and denied visits by her parents as punishment.

In the documents, Mr Levitt alleged the 13-year-old had been handcuffed and shackled and routinely subjected to rough treatment, including having her head banged against a wall.