An independent inspector of a juvenile detention centre passed on the chance to expose and eradicate systemic brutality against children, a leading human rights lawyer has said.

George Newhouse said the review by Western Australia’s independent inspector of custodial services “downplayed” the allegations raised by Amnesty International about two young detainees, including that they were held in “solitary confinement” for prolonged periods.

“In doing so he has passed up the opportunity to expose and eradicate the systemic brutality and child abuse that exists in the WA youth detention system,” said Newhouse.

Allegations – which included the solitary confinement of teenage detainees, strip searches, excessive restraint, food used as punishment, and lack of contact with carers about boys self-harming – were reported by Guardian Australia, and investigated by the inspector, Neil Morgan, whose final report was delivered last month.

Morgan found suggestion of many allegations but a lack of evidence. He found Banksia Hill detention centre did breach international standards by “probably” holding the two teenagers in “conditions that constituted solitary confinement” for periods of 10 days.

But he said the majority of allegations – including that one boy self-harmed more than 100 times and was regularly strip searched – were not substantiated.

Newhouse responded that this did not mean the allegations did not happen.

Morgan’s report said records kept by Banksia Hill in the weeks immediately following the move to the intensive support unit were insufficient or non-existent.

“If that evidence had been available and had the past recommendations of the inspector been implemented, the inspector would have been able to make more conclusive findings,” Newhouse said.

“At the end of the day, the inspector simply could not substantiate many of the allegations made by two of the boys because his own past recommendations have not been implemented. In doing so he has avoided the opportunity to investigate how the organisational culture in the WA Department of Custodial Services has led to cruelty and abuse of children in Banksia Hill.”

Morgan’s report found the confinement of the two boys breached the Mandela rules, Havana rules and the convention against torture. It did not however, technically breach WA law “because the boys were managed under individual case plans, which circumvented rules around solitary confinement and “arguably undermine the spirit of the [law]”.

Newhouse said it was “appalling” that WA laws fell behind international laws and standards.

“In Western Australia the rules concerning the confinement are set out in the Young Offenders Act and regulations,” he said.

“Under those laws, confinement is permissible as a way of dealing with a detainee who has committed a detention offence or to maintain good governance, order and security in the centre. There are no time limits on how long a child can be confined in WA, just that it must be done with the permission of the superintendent.”

The WA corrections minister, Fran Logan, had called on Amnesty to apologise for “grievous slurs” against staff at the centre.

Logan defended the confinement of the boys as “the prudent thing to do” after the incident.

Logan, who ordered the investigation, has maintained no juveniles are held in solitary at Banksia Hill and said they had “access to the same privileges as other young people”.

The union representing Banksia Hill staff also rejected the allegations.