Detainees at Victoria's largest maximum security jail were forced to remain in their cells after prison officers walked off the job over a months-long pay negotiation impasse.
More than 200 guards at the Port Phillip Prison in Truganina, about 20km west of Melbourne, undertook stop work action and rallied outside the facility from 7.30am to 1pm on Tuesday.
The protected industrial action was timed to coincide with the day shift changeover with the disruption lasting into the afternoon, with the prison housing more than 1000 detainees.
Apart from those who needed to attend court, prisoners remained locked down in their cells during the action, a Community and Public Sector union spokesman said.
Striking members hoped the action will reinforce workers' rejection of the most recent pay offer of 3.8 per cent by prison operator G4S.
But the union said the operation remained silent on their response and on any improvement to their pay offer after the action.
The union's Victorian assistant secretary Wayne Townsend said his members worked hard in difficult circumstances and just wanted a fair deal.
"G4S is a large multinational company and largely offshores its dollars to its overseas-based parents and needs to put its locals first," he said.
Union members will give notice on further action that will limit prison operations which will start on Saturday and further stoppages next week.
The prison operator withdrew the 3.8 per cent pay offer after it was voted down and reverted back to an original offer from November of a three per cent pay rise.
The union argues this would be below living costs and failed to match the Fair Work Commission's decision on Monday to lift the minimum wage by 3.75 per cent in July.
The two groups have been locked in protracted negotiations since late last year and workers had rejected two previous offers from the company.
"G4S seem prepared to force this extended disruption on inmates and needs a change of heart," Mr Townsend said.
"Workers need job assurances and an improved pay offer."
The prison will have contingency plans and a G4S spokesman said the safety and the wellbeing of employees and prisoners was the number one priority.
"G4S is disappointed that the bargaining representatives have rejected our latest offer but we will continue to negotiate with them and hope to reach an amicable agreement," he said.
Unionised officers at the prison have been protesting since May 21 by not answering management calls, emails and text messages and ignoring orders unless there was a risk to safety.
There has also been an indefinite ban on officers completing case management paperwork, except if related to prisoner wellbeing, and workers signing onto shifts during rostered hours.
The union ramped up pressure on Thursday with workers locking down prisoners 30 minutes earlier every day to allow for unionised officers to discuss bargaining activities and members encouraged to not comply with the media policy and speak to journalists.
