"Eleventh-hour meetings held between the PSA bargaining team and Ministry of Justice to reach a resolution failed to get any fundamental movement in their pay offer," PSA National Secretary Glenn Barclay said.
"Our members work hard to keep the justice system working and we are seeking an outcome that values all staff and ensures they are reasonably paid.
"The Ministry offered the third-lowest average salary in the public sector last year and we believe this is being worsened," he said.
Barclay said the average female employee at the ministry is paid 15 per cent less than the average male worker, a larger gender pay gap than the average public sector department.
"The Ministry's own engagement survey shows that only a third of staff feel valued for the work that they do, with a clear impact on recruitment and retention issues," he said.
PSA members would also "work to rule" from 4pm today until 4pm on October 19, with an overtime ban, only working contracted hours and taking common breaks.
"Public servants deserve better than this, and [today]they will tell their employers that," Barclay said.
The Government requires all public service employers to end the gender pay imbalance by 2020.
Ministry of Justice chief executive Andrew Bridgman said bargaining on a new employment contract with the PSA would be ongoing for several months.
"The Ministry has offered a 5 per cent increase over two years, in line with other agreements in the public sector, but the PSA is seeking more than 13 per cent.
"The Ministry remains open and committed to reaching a negotiated settlement and we're ready to meet at any time," Bridgman said.
The Ministry's contact centre (0800 268 787) will continue to operate during the strike action but callers were warned to expect delays.