The Public Service Association is taking legal action against Oranga Tamariki over its handling of proposed job cuts.
Oranga Tamariki is looking to axe more than 400 roles, following a government directive to slash spending.
But the PSA claims Oranga Tamariki breached its collective agreement by not properly involving the union before putting together a proposal for job losses.
On Tuesday it filed proceedings in the Employment Relations Authority in a bid to put a hold on the process.
"We are asking Oranga Tamariki to pause the change process and for no dismissals to take place as it will take some time to work through the provisions in the collective agreement," said Assistant Secretary Fleur Fitzsimons.
The PSA had consistently raised concerns about the scale and nature of the changes, including its impact on staff and their whānau, and on the services Oranga Tamariki provides, she said.
The legal action against Oranga Tamariki is similar to the case the PSA successfully took against Ministry of Education, Fitzsimons.
"Though there are some differences, the Oranga Tamariki and MOE collective agreements both provide a role for the union in proposing the nature of change and reaching agreement on recommendations to management who must then take them on board.
"In both ministries, the employers have not complied with this requirement."
The ERA ruled last week that the Ministry of Education had to work more closely with the union before deciding on its cuts.
Given that decision, the PSA was hopeful they would not need to argue the Oranga Tamariki case before the ERA.
"We have asked for an order from the Authority to attend mediation in an attempt to settle this dispute," Fitzsimons said.
An Oranga Tamariki spokesperson said they were aware of the legal proceedings lodged by the PSA.
They said the agency was taking advice and had no further comment at this time.