A total of 260 probation officers have begun industrial action that could cause delays in the court and parole system.
The dispute is over the Department of Justice's decision to recruit a new director of the Probation Service.
The probation officers, who are represented by IMPACT, say the new director must have a social science qualification. But the Department of Justice says although such a qualification was desirable, the priority was to find a candidate with suitable administrative skills.
IMPACT gave the department a one-week deadline in which to promise the next director would have a social sciences qualification. Otherwise, stronger action may be taken by the officers who work in prisons, courthouses and in the community.
The officers withdrew their co-operation from the Parole Board and were refusing to provide risk assessments of offenders seeking parole. They will also stop giving out-of-hours advice to drug task forces.
If their demands are not met, parole officers could refuse to make court appearances or provide offender assessments for judges deciding how to pass sentence.
At a weekend, the union's strike committee rejected a proposal by the Department of Justice to appoint an existing staff member as an acting chief for 12 months from June, by which time the new director must be in place.