More than 150 officers from Parklea Correctional Centre in northwest Sydney took industrial action on Friday over what they say is the lowest pay in the country in a jail with some of the worst assault records in NSW.
It is the second strike in as many months.
Community and Public Sector Union NSW assistant general secretary Troy Wright said officers at the privately run prison suffered the highest number of serious assaults in the state.
He said the number of assaults among inmates in the year ended June 2022 was the second-highest in the state.
About 30 officers holding placards calling for "a safer workplace" formed a picket line in front of the jail on Friday morning.
Among the reasons pushing them to take industrial action, Mr Wright cited an inmate believed to have set fire to the prison in 2021, causing $8 million in damage, as well as reports of officers being assaulted.
"We know of prison officers being king-hit from behind, having shivs held to their throats, being punched in the face and having faeces thrown on them," he said.
Mr Wright blamed MTC - a United States-based firm that also runs a detention centre housing refugees on Nauru - for failing to protect officers at Parklea, one of only two private jails in NSW.
The officers are negotiating a new enterprise agreement with their employers, MTC Australia, and are seeking a 3.5 per cent wage increase.
They claim to be stuck on an offer from their employer of 2.5 per cent.
"The business model of MTC is to make their money from their own prison officers," Mr Wright said.
MTC disputes the union's claims, saying it has been negotiating "in good faith" for more than six months, offering staff a 12 per cent pay rise across the next four years.
"We have been negotiating with the union in good faith since May last year and met with them as recently as last week,"Â an MTC Australia representative told AAP on Friday.
"We will continue to work with them to reach an agreement as we are keen to give our staff a pay rise."
It says it has a contingency plan to ensure enough staff oversee inmates while the strike is ongoing.
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