Youth justice centre inmates inside the adult Barwon Prison have been exposed to capsicum spray, a court has heard.

Dr Ian Freckelton, QC, for the children, told the Supreme Court on Monday that detainees - many of whom had at times been held in isolation for 23 hours a day - had also been exposed to capsicum spray through the ventilation systems in the Grevillea unit of the maximum-security prison.

Parkville Youth Justice Centre rioters. Credit:Jesse Marlow

These, Dr Freckelton said, were examples of a facility "incapable" of meeting their human rights in detention.

Details about the conditions in the unit were revealed on the first day of a fresh legal challenge against the state government's move to re-classify it as a youth justice centre.