Forty-five drivers for New Zealand Bus walked out of the Glenfield depot around 5am this morning protesting about pay and roster issues.
The drivers' transport unions, First Union and the New Zealand Tramways and Public Transport Union, both said they had no knowledge of the protest until after it was happening.
The president of the Auckland Tramways and Public Transport Union, Gary Froggatt, said the drivers returned to work around 9am this morning and the affected North Shore bus services were running again.
He said the protest followed NZ Bus "arrogantly, and without consultation," cancelling a scheduled mediation hearing in late January.
Mr Froggatt said the drivers were protesting about the shift lengths, roster issues and unpaid hours in the middle of the working day.
First Union said the drivers were frustrated the company was refusing to mediate with them.
In a statement, NZ Bus said it understood the protest related to shifts and roster changes that took effect last year.
NZ Bus said "the protest went ahead even though there is already a meeting organised for next week to discuss this and other workplace matters with the workers and their union representatives".
Gary Froggatt said the company's management was now threatening legal action against the drivers.
He said the New Zealand Tramways and Public Transport Union, alongside the First Union, would be speaking with NZ Bus management after this morning's protest.
Auckland Transport said some North Shore services were affected this morning but the protest did not affect school buses and the Northern Express.
It said following services were affected: 82, 101, 105, 110, 129, 801, 802, 805, 806, 807, 814, 842, 843, 923 and 924.