First Security says it's collaborating with union members to make sure work escorting prisoners and providing security at courts is covered during an overtime ban.
Prison escorts and court custodial officers are responsible for transporting and supervising prisoners between prisons, courts, hospitals, and other locations.
They also provide security and custodial services at courts, managing the safety of judges, staff, witnesses, and the public.
E tū, the union representing the security guards, said members have rejected a pay rise which would take their base rate to $30 an hour.
It said the offer was still lower than what other agencies paid.
The union said the overtime ban was the first step in industrial action. As workers did more than 10 hours a day on average, this was expected to have a significant impact on the company's operations.
E tū said First Security was also refusing reasonable claims for improvements to sick leave, annual leave and overtime.
Union delegate Tai Henry said the guards were undervalued.
"All we want is decent conditions and pay and to be treated fairly," he said.
"We know our job conditions are worse than at other agencies where people are doing similar work to us. But we get worse pay and longer hours. It's not right. It's not fair."
A spokesperson for First Security said the offer represented a minimum 10 percent pay rise, along with significant additional benefits.
"We are aware of comparisons with other government agencies and are continuously reviewing our compensation to align with industry standards in both the private and public sectors.
"First Security values its prison escort and court custodial officers and recognises their vital role in public safety. We have been negotiating in good faith to ensure our staff receive the best possible offer for their unique private sector role," the spokesperson said.