Unions want a pay rise for Victoria’s juvenile prison staff to reflect the dangerous nature of their work.

Assaults on staff in Victoria's juvenile prisons have doubled in two years, playing a big role in a blowout in the Government's WorkCover insurance premiums, which now cost more than $4.2 million a month.

The Community and Public Sector Union (CPSU) wants staff to receive a 3.5 per cent pay rise to adequately reflect the dangers of the job.

There were 130 assaults on staff at Malmsbury and at the Melbourne Youth Justice Centre in Parkville last financial year - more than one assault every three days.

Most of the workers at Malmsbury and Parkville earn between $26 and $34 an hour.

“They are one of the lowest paid occupations across all public service entities,” CPSU branch secretary Karen Batt said.

“We say given the complexity and the nature of their work, they certainly deserve a lift in pay.”

WorkCover insurance premiums for the Department of Justice have more than doubled since youth justice was made a part of it in 2017. The premiums are up from $1.6 million a month to $4.2 million a month last financial year.

The union said the Victorian Government is paying the premiums without addressing their source.

“They are simply providing a source to get compensation, rather than sitting down and investing much more in the health and safety and the training of the staff who are currently confronting these very violent situations,” Ms Batt said.

Youth Justice Minister Ben Carroll appears to agree that violence against staff is unacceptable.

“We have invested more than $1 billion to overhaul our youth justice system, delivering more and better trained and equipped staff and upgrading infrastructure,” he said.

The Government is also imposing tougher consequences for young people who assault staff.

A third youth justice centre west of Werribee is scheduled to open in 2021, which the government says should help ease overcrowding in the other facilities.