All South Australian prisons are in lockdown a day after a prisoner was allegedly assaulted, but the Department for Correctional Services (DCS) says the industrial action was pre-planned.
The Public Service Association SA said on Sunday an inmate was taken to hospital after that he was left barely recognisable.
The latest alleged assault followed in recent months.
The union's general secretary in SA, Charlotte Watson, told ABC Radio Adelaide the latest prisoner-on-prisoner incident was "like the straw that broke the camel's back".
"They [union members] held an urgent stop-work meeting this morning and that happened statewide at all of our prisons and they've determined to do a 24-hour work stoppage today, which means there won't be staff in Yatala today, which means the prisoners are in their cells, unfortunately," she said.
"It's a lockdown … and that's been voted on this morning."
Charlotte Watson gives a speech at a rally to demand high wages last month. (ABC News)
The department confirmed a lockdown was underway in all prisons as a result of industrial action by the union.
"The industrial action by the PSA was organised prior to yesterday's alleged assault at Yatala Labour Prison," DCS said in a statement.
"The state government is continuing to negotiate in good faith with the PSA and it is unfortunate the PSA has undertaken this action.
"South Australia's corrections officers are some of the most hard working and highly trained in the nation."
Ms Watson said during the lockdown, prisoners would be "contained" in their cells until prison officers returned.
"It means that it will need the upper managers of DCS, who usually wouldn't be going down into the wings, they'll be going down into the wings to be providing prisoners with food through the traps in the doors," she said.
"This is the result of several brutal attacks that have occurred in our prison system."
The union said the latest incident happened in the prison's B-division, where an inmate allegedly assaulted another before two others joined in the attack.
SA Police is investigating the incident, but no charges have been laid.
The union, which is pushing for higher wages amid enterprise bargaining with the SA government, said there was a direct link between prison staff's safety and their pay.
"They feel they've got no other choice at this point," she said.
"They've been screaming out for support and better pay for quite some time now."