Hundreds of bus trips in Auckland have been axed this morning and the union says its industrial action may be repeated throughout the week.
Drivers employed by bus operator NZ Bus went on strike at 4am and were due to resume work again at 8am.
NZ Bus is one of seven operators providing bus services on behalf of Auckland Transport.
The drivers want a higher hourly rate to bring them in line with their counterparts in Wellington, Christchurch and Dunedin.
The New Zealand Tramways and Public Transport Union is one of two unions representing them.
National president Gary Froggatt said drivers had not had a pay increase from NZ Bus for over 10 years. All they had received was cost of living increases.
A formula had been given to the company to enable drivers to be paid $30 an hour.
Drivers were being paid $26.76 at present, and have been offered an increase to $28 with a $30 rate to take effect next year.
Froggatt said the union's formula would increase the hourly rate faster - with quarterly pay rises.
The union had asked for immediate mediation from last Thursday, and decided to take strike action today because the company had not agreed to that request.
He defended the drivers taking industrial action at such short notice, saying the union only had to give 24 hours notice.
Asked how likely it was the mediation would result in a deal, he said: "I'm always optimistic but I can't give any guarantees."
If there was no deal today, there might be more stoppages for the rest of the week, he said.
"If we reach settlement today, then we will call off any further action."
It was school holidays so school bus services would not be affected, however, the union did not want to see children and teens wandering the streets in search of public transport, he said.
"If we reach settlement today, then we will call off any further action" - NZ Tramways and Public Transport Union president Gary Froggatt
AT 'optimistic' for settlement
Auckland Transport public transport services general manager Stacey van der Putten said thousands of commuters would have had their services disrupted, however, it was one operator affected out of the seven used by AT.
For the morning peak it was likely the industrial action would have hit 7 - 8 percent of the city's bus services.
It was disappointing the action had been taken and AT had tried to ensure its Journey Planner and website were up-to-date so disruption was minimised.
"Both parties are very motivated to get this resolved ... so we are hopeful for a positive outcome and we'll be talking to our operator to understand how things eventuate today."
AT took the view that all public transport bus drivers across Auckland should be paid $30 an hour by the end of the year.
Each operator had different pay structures but "on average those rates will be met within what they have proposed or above [$30]".
"Both parties are very motivated to get this resolved" - Auckland Transport public transport services general manager Stacey van der Putten