Three inmates at a troubled youth prison in central Victoria have been injured in a violent clash that involved stealing a guard's keys.
The detainees were taken to hospital in a stable condition after suffering upper body injuries at Malmsbury Youth Justice Centre on Tuesday afternoon.
One of the three youths has since returned to hospital for further treatment, with a fourth given first aid at the scene.
No staff were injured but police were on site to assist after reports of an affray inside the facility, located north of Melbourne near Kyneton.
"This incident will be subject to a thorough investigation and has been referred to Victoria Police," a Department of Justice and Community Safety spokeswoman said in a statement on Wednesday.
"The safety and wellbeing of staff and young people is always our number one priority.
"We are continuing to support staff and young people involved in the incident."
Detectives are investigating several reported assaults, police said.
Attorney-General Jaclyn Symes, who was briefed on the incident, said inmates were able to get a set of keys from a guard.
She was not aware of the prisoners' motivation but said their behaviour was completely unacceptable.
"There'll be consequences as a result of that," she told reporters at state parliament.
Malmsbury is slated to close by the end of the year, with detainees being transferred to the Melbourne Youth Justice Centre in Parkville and a new $420 million facility at Cherry Creek.
Operational since 1965, the Malmsbury centre has battled problems for years with inmate riots, staff shortages and escapes.
The new Cherry Creek centre is purpose-built and features an intensive intervention community, specialised primary health centre and two mental health units.
It will meet the recommendations of a review of the state's youth justice system released in mid-2017.
