Prison officers at Parklea jail in Sydney's west are striking for three days as they continue a campaign for more pay and better safety at work.
It's the third time the officers, who will rally outside the Parklea Correctional Centre at midday on Wednesday, have walked off the job in as many months.
Community and Public Sector Union NSW assistant general secretary Troy Wright says the jail near Blacktown has the worst record for serious assaults on prison officers in NSW and the second worst rate of assaults amongst inmates.
Officers at the maximum security men's prison were the worst paid in the country and were consistently forced to work while understaffed, especially on weekends, he said.
The jail is privately run by US multinational MTC.
"Imagine being so desperate for change in your workplace you would forgo three days pay in the hope someone would listen," Mr Wright said.
In the past few years, there has been a riot and two fires at the jail, one of which caused $8 million in damage.
In the past year, there have been three deaths at Parklea Prison.
"In the last few weeks, a prison officer was king hit from behind and we know of officers having shivs held to their throats and being punched in the face," Mr Wright said.
"Understaffing is chronic and causing a lot of these problems, we know this prison is short-staffed each and every day."
Some areas of the jail had 480 prisoners, which are meant to have 28 officers looking after them but routinely have just 17, Mr Wright said.
"Because MTC doesn't pay penalties for working weekends, they save money by making their staff work as much as possible on Saturday and Sundays when they should be spending time with their families."
The union wants an increase in staffing levels and less weekend work.
Comment is being sought from the prison.
