Bus drivers will protest outside Auckland Transport offices tomorrow, against roster changes they claim could leave them fatigued and put passengers at greater risk.

The changes are proposed by their employer, NZ Bus, but driver unions said the company was imposing them in response to new weekend schedules the council body intends introducing on July 19.

They have filed an urgent claim in the Employment Relations Authority, alleging a breach of the drivers' collective contract with the company, after a failed mediation bid.

Despite a planned protest outside Auckland Transport's offices opposite Britomart at noon tomorrow, the council body said the dispute was an operational matter between the drivers and NZ Bus on which it does not intend commenting.

NZ Bus chief northern operating officer Shane McMahon said the company had a set amount of weekend work it was contracted to perform, and wanted to share it across all drivers to be fair to all.

That will end a seniority exemption now extended to those with longer service.

"We believe sharing that weekend work across all employees is fairer and means that some of the younger drivers with families are not required to work an excessive number of weekends," Mr McMahon said.

"We appreciate there will be vocal resistance from a few drivers who are seeking to maintain their position, but it is about doing what is right for the majority of our people."

But Tramways Union president Gary Froggatt said they would affect about 600 drivers at all levels of seniority from five depots, requiring them to work clusters of weekends rather than one in three as many do now.

"It means they may not be able to go to church -- a lot of drivers have religious affiliations," he told the Herald.

He said the company also intended increasing unpaid time in the middle of daily split shifts to four hours, from three hours now.

First Union organiser Rudd Hughes said the company had consistently ignored feedback from drivers, who needed rosters "which will keep passengers safe and provide a decent work-life balance."

He said the two unions have provided alternative roster proposals, but the company had rejected those.