Thousands of Auckland commuters will have to wait twice as long for peak train services tomorrow after a reshuffle of the timetable to ensure train drivers don't work overtime.

Train services in Auckland will be affected for the next three weeks as unions take industrial action.

Its start coincides with the start of the busiest period for public transport as students return to university.

About 10,000 people catch the train during peak hours from 6.30am to 8.30am and 4.30pm and 6.30pm, Auckland Transport's chief transport services officer Mark Lambert said.

During those hours from tomorrow trains on the Southern, Eastern, and Western lines would come every 20 minutes instead of every 10, he said.

"But additional carriages will be put on ... services during those times will be a lot busier than normal."

Extra buses would also be running, Mr Lambert said.

The Onehunga and Pukekohe weekday train services will have the usual timetable.

The change was made after the train workers said they won't be working overtime, Mr Lambert said.

Bus and ferry services are not affected.

Rail and Maritime Union said it had safety concerns about a proposal to give train drivers the responsibility of opening and closing doors.

Union organiser John Kerr said the train manager should be responsible for the doors.

"A lot of Auckland station platforms are curved, so even if the driver's got mirrors and got cameras they're not going to be able to see if there's a passenger that's stuck in a door, or even worse, has gone between the platform and the train."

Mr Kerr said the union would meet with Auckland Transport on Wednesday.

Auckland Transport said the change would allow for more train officers to manage security and incidents onboard.