First Union is accusing NZ Bus of breaching the Employment Relations Act by inviting staff to discuss the bargaining process in a series of meetings without the union's involvement.
Hundreds of NZ Bus staff in Auckland started working to rule yesterday to protest against what they said were long hours, bad conditions and low pay.
First Union organiser Rudd Hughes said the company was trying to side-step the bargaining process.
The union was instructing its members not to attend the meetings which were due to start tomorrow, he said.
NZ Bus could not be reached for comment.