A group of 17 youths were last week moved from Perth's Banksia Hill detention centre to a standalone facility at the nearby Casuarina maximum-security prison.
Officials say the boys, aged as young as 14 and mostly Indigenous, have been destroying property, escaping from their cells, assaulting staff and harming themselves.
No timeline has been set for their return.
Family members have revealed the boys were chained by their hands and feet while being relocated on a bus.
Premier Mark McGowan on Monday said authorities had been left with no choice because of the detainees' "extreme" behavioural issues.
"Under the law, that is permitted if required," he told reporters.
"They had to do it because of some of the activities, the behaviours that some of these offenders are exhibiting.
"It's very sad that this is occurring but we have to protect the staff. The staff can't be under threat. We're doing our best to deal with what are some very difficult issues."
A relative of one of the youths said the family had been given no information on how long he would stay at the new facility.
She alleged the boy had been asked to take his night-time sleeping pills before the morning transfer.
"That disturbs me ... because being in shackles and being sedated could be quite dangerous," she told AAP.
A spokesman for WA's Department of Justice said young people at Banksia Hill were never sedated.
The Office of the Inspector of Custodial Services said it had observed the transfer and found it to be well coordinated and respectful to the detainees.
Authorities have promised the boys will be kept away from adult prisoners in safe and secure units while repair works are completed at Banksia Hill.
But advocate Megan Krakouer, from the National Suicide Prevention and Trauma Recovery Project, said it was possible for men to speak to the boys through a fence.
"They are having interaction with the adult prisoners in there, that's the reality," she said.
Department of Justice official Christine Gibney said the transfer was "not something anyone did lightly".
"This is an extraordinary action and was necessary because the young people are not safe or secure at Banksia Hill," she said.
"Neither are the other young people who are housed there or the staff."
The independent Inspector of Custodial Services in April found some boys at Banksia Hill were spending as little as one hour a day outside their cells, in violation of their human rights.
About 600 past and present detainees have signed up for a planned class action led by Levitt Robinson Solicitors, alleging they suffered inhumane treatment, were excessively restrained and denied access to education.
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