Environment Canterbury (ECan) workers are walking out the door on Monday over salary issues.
Nearly three-quarters of union members claim they are paid less than the standard rate for their positions.
Public Service Association organiser Angela Collier says it's unacceptable.
"Some members have experienced slippage of up to 4 percent over a three-year period, with stalled wages failing to keep up with the market."
The members are striking from midday, following what they call "protracted negotiations at ECan and fundamental disagreements over the current salary system". They want a 4.9 percent pay rise, ECan said.
Other concerns the union has include exclusion of recognition of Te Tiriti and ECan obligations to staff.
"ECan have claimed they have a legal opinion about not putting in a clause in the collective agreement recognising Te Tiriti and obligations to staff," said Ms Collier. "We have asked for their legal opinion on this, and they have failed to provide it."
ECan told RNZ at the weekend it has offered staff a 1.1 percent pay rise.
"The PSA has called for their members to take industrial action, and we respect their right to do so. We will be continuing discussions with the PSA to reach resolution," a spokesperson said.
"We are in the middle of good faith bargaining and cannot comment further."