Prison officers across NSW are striking over job cuts and unsafe working conditions, with their union claiming lives are at risk.
The industrial action kicked off at Long Bay jail on Friday morning, and by midday, the Public Service Association estimated up to 900 staff had walked off the job.
The PSA's Prison Officers Vocational Branch chair Nicole Jess warned the number may reach 4000 as staff at Goulburn, Berrima, Silverwater and Dillwynia prisons held stop-work meetings and began to strike.
"It is a statewide walk out," she told reporters in Sydney.
PSA general secretary Stewart Little said the government's so-called benchmarking process was merely a guise for job cuts at a time when the state's prison system is in crisis.
"This is not about efficiency," he said.
"This is ludicrous, this is dangerous and this is going to place officers' lives at risk."
Mr Little said the inmate population was fast approaching 14,000 in a system only designed to hold 11,000.
He said assault rates on officers had nearly trebled in three years.
Corrective Services NSW argues benchmarking will raise standards and improve accountability by measuring safety, security and inmate rehabilitation outcomes.
It involves reduced staff in some areas and extra staff in others, based on risk assessment.
At Long Bay, 90 jobs were on the chopping block, the union claimed.
Commissioner Peter Severin urged officers to reconsider the strike, pointing out no decisions had been made on job changes.
"We have made it clear we will not implement any changes without a careful risk assessment and safety and security remain paramount," he said.
But Ms Jess said staff had already been through a three-month process including risk assessments, and the department was simply coming back with the same numbers.
"They're not listening, they're not working with us," she said.
Opposition leader Luke Foley said prison officers haven't been getting the government support needed to deal with "the most dangerous people in our society".
"Prison officers face a risk to their lives on the job every day," he told reporters in Sydney.
"They are ill-equipped, they're under-resourced, and now they face job cuts and having to do their dangerous jobs with less and less support. So I support what our prison officers are doing."
Prisoners were kept in their cells on Friday and confusion reigned in many courtrooms, including at Sydney's Downing Centre, where inmates weren't able to appear via video link.
Asked whether the strike could continue into next week, Ms Jess said she would be led by members.