Thousands of Auckland schoolchildren have been spared mass strandings tomorrow after a promise by bus unions that drivers will return to duty from a big stopwork meeting.

Bus operator Stagecoach was alarmed by ominous advice from the unions on Friday that "some members may not return to work" after a meeting from 11am to 3pm.

The company said it was particularly concerned about its ability to deliver children home from school.

But it found out yesterday that the meeting will run from noon until about 2pm, and combined unions advocate Gary Froggatt said he had given Stagecoach an assurance that "if we take children to school in the morning we will get them back home in the afternoon".

Stagecoach expects about 33,000 of its 160,000 daily passengers to face disruption once it starts removing buses from the road progressively from about 9.30am.

Industrial action by the drivers in support of a pay rise to $16 an hour and improved conditions such as morning and afternoon tea breaks will in fact start today with a "work-to-rule" campaign.

Mr Froggatt said this would mean the unions would encourage drivers running late because of congestion or other reasons not to risk receiving speeding tickets by trying to catch up.

This could mean some extra delays for passengers.

The unions want times adjusted to ensure drivers have long enough at the end of bus runs to check for lost property and rubbish in their vehicles and stretch their legs.