The Community and Public Sector Union's Victorian secretary Karen Batt said talks had deteriorated with prison operator G4S and she urged the state government to intervene. The union says G4S has offered a 2.5 per cent pay increase.

"G4S must improve their offer to employees and be genuine if this dispute is to be resolved," she said. "Talking for months and going round in circles isn’t being genuine."

But G4S correctional services director Craig Castle said the parties were negotiating in good faith.

"We believe our staff will continue to operate in a way that ensures the safety of the people in our care, their colleagues and the overall integrity of the facility," he said.