The Public Service Association has filed a case seeking equal pay for all social workers.
It has filed the case under the Equal Pay Act 1972, arguing that social workers at Child Youth and Family do not receive equal pay because of their gender.
It is also seeking a determination on what they should be paid.
The union's national secretary Erin Polaczuk said social workers were underpaid because more than 70 percent of them were women.
Last month the government announced a working group on the gender pay gap and unions agreed to put legal action on hold until March to allow it to report back.