Union members working in courts and tribunals at the Ministry of Justice are taking industrial action on Wednesday by refusing to do overtime.
Pay talks between the ministry and the Public Service Association have broken down.
The PSA says Wednesday's work-to-rule by 1700 members will escalate into strikes within four weeks.
The union says Justice employees are paid 6% below the public service median pay rate.
The ministry says closing that gap would cost $100 million over three years.