Teens who rioted at Victoria's highest security youth prison had been moved there because they caused problems at other prisons.
About a dozen youths aged 16 and 17 confronted prison officers at the Malmsbury Youth Detention Centre at 7pm on Saturday, rioting for several hours.
Victoria Police and the Health and Human Services Department decided it was too dangerous to send anyone into the Deakin unit to quell the riot.
"They let the kids burn themselves out, was the expression used to me," Community and Public Sector Union spokesman Julian Kennelly told 3AW on Monday.
Children's Minister Jenny Mikakos said the riot was unacceptable.
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"Unfortunately some of the young people involved in the incident on Saturday night had been involved in previous incidents at Parkville (detention centre), and had been moved to Malmsbury," Ms Mikakos told reporters.
"It is very disappointing that this behaviour occurred at what is designed to be a higher security facility."
Ms Mikakos said the riot caused "thousands of dollars" worth of damage, and it would be investigated.
There were no reported injuries to staff or prisoners at the prison, which is about 100km northwest of Melbourne.
Mr Kennelly said staff were concerned about a pattern of bad behaviour in Victoria's youth detention centres.
"I reckon I've received a text message each Saturday night for the last three months that something's happened at Malmsbury or Parkville (detention centres)," Mr Kennelly said.
Victoria has experienced a wave of young recidivist criminals committing carjackings and home invasions, leading to more being held on remand for violent crimes.
Ms Mikakos said Victoria Police had done a good job arresting young offenders but it had led to youth prisons holding a complex group of offenders.
"There is a challenging group of young people in our youth justice facilities," she said.
