A Parkville Youth Justice Centre worker was taken to hospital with facial fractures after he was assaulted when he tried to break up a fight on Sunday afternoon.
Paramedics were called to the facility just after 2.30pm, after reports that a worker who tried to end an altercation between inmates had been attacked.
The man was treated for upper body injuries and transported to hospital in a stable condition.
Police media spokesman Sergeant Scott Cameron said the force was investigating the assault, with all offenders identified as they remain in custody.
"Police have been told a group of inmates were involved in a physical altercation about 2.30pm," he said.
"A staff member attempting to break up the fight was struck and is being treated for non-life threatening injuries."
A Department of Justice and Community Safety spokesman confirmed the worker was assaulted on Sunday afternoon.
"Violent behaviour or assaults at youth justice centres are absolutely unacceptable, and the safety of staff, young people and the community is of the utmost importance," the spokesman said.
The Community and Public Sector Union's Victorian branch assistant secretary Wayne Townsend said repeated assaults on youth justice centre workers cannot go on, with the injured officer suffering facial fractures.
"It was six versus one. We intervene to stop them killing someone else and our officer ends up in hospital," he said
"It's the same cohort, and they're clearly not secure from other young offenders where they are.
"Somewhere else has to be found for them to be located while we await Cherry Creek."
The union has called for an "interim" facility to be brought online before the new Cherry Creek youth justice facility is built in 2021, west of Werribee.
The union has previously reported there were 311 assaults on staff at youth justice centres in the first 120 days of this year, ranging from physical assaults, to spitting, to threats of violence and verbal abuse.
In October, two 18-year-olds and a 19-year-old were charged with intentionally causing injury, affray and assaulting an emergency worker at the Malmsbury Youth Detention Centre.
The facility was locked down following the alleged assault, with movements restricted for inmates and staff.
A spokesman for the Department of Justice and Community Safety said laws had been introduced to crack down on violent behaviour in the youth justice system.
These include a presumption that offenders who assault prison workers will serve their sentences on top of an existing period of detention.
"New laws are now in effect that strengthen consequences for young people who assault youth justice custodial workers, damage property, and escape or try to escape," the spokesman said.