The president of the union's provincial wing, Danny Legere, issued a statement Thursday saying the Conservative government is violating the Public Service Labour Relations Act.
The union says the government is violating the act by changing the conditions of collective agreements after five of the union's locals indicated they were prepared to negotiate renewed deals.
Legere says the union is taking legal action because it wants to protect its defined-benefit plans.
Earlier this month, New Brunswick Finance Minister Blaine Higgs said the government had gained support for the changes from four of the five unions that represent the civil service.
Among other things, the government wants to move to a shared-risk model for pensions to address a $1-billion pension deficit.