Potential disruption looms again for tens of thousands of commuters as Auckland's main bus operator ponders whether to retaliate against a new threat of industrial action by 900 drivers and cleaners.

NZ Bus refused to be drawn yesterday on the likely impact for about 70,000 daily passengers of a work-to-rule by drivers from Thursday, including five-minute toilet and exercise breaks between trips and a ban on operating defective buses.

But the Infratil subsidiary said passengers should start thinking now about other ways of getting to work, and apologised for the renewed uncertainty raised in its five-month pay dispute with its unionised workforce.

"We apologise sincerely for this - we are bemused by the unions' desire to initiate industrial action when the option of resolving this in facilitation [by the Employment Relations Authority] exists," said operations manager Zane Fulljames.

Mr Fulljames would not indicate whether the company was considering locking out the drivers, as it threatened to do last month, before an agreement with the four bus unions to return to mediated negotiations averted a regional transport meltdown.

"We are taking some time to consider potential impacts of the unions' notice of industrial action on our customers," was all he would say about that.

"We have an obligation to provide our customers some forward advice to start thinking about alternative ways to get to work on Thursday while we assess that situation."

Although acknowledging the school holidays would reduce pressure on the bus network for the rest of this week, Mr Fulljames noted most youngsters were due to return to classes on Monday and the university term had already begun.

The Auckland Regional Transport Authority is considering contingency plans including arranging extra services by other bus operators and rail company Veolia Transport.

But combined unions spokesman Karl Andersen last night urged the bus company not to use "a sledgehammer" in response to what he saw as low-level industrial action by the drivers and cleaners.

"My assessment is that it will cause some annoyance to them [the company] and some disruption to services but we think it will be very minor," he said.

Despite that assessment, he acknowledged uncertainty over how many buses may be deemed defective by their drivers on the day.

"I don't know whether it is going to be six, 20, 100 or 200 buses that don't have working radio telephones," he said.

Among the proposed bans are refusals to operate buses without effective radio communications, certificates of fitness or any safety defects "the driver is aware of".

Neither was Mr Fulljames able to indicate how many buses may be deemed non-roadworthy.

"Let's be clear - if notice of industrial action is purely work-to-rule, that's somewhat innocuous in terms of its content," he said.

But he added: "You wouldn't issue notice of industrial action if there wasn't potential for disruption.

"What we need to determine is what that level of potential disruption could be."

The company is offering hourly pay rises of about 70c an hour now, to be followed by 50c next year and 60c in 2011.

These levels, it says, are similar to settlements reached by the unions with other Auckland bus operators.

But Mr Andersen said wages in other fleets had started higher than at NZ Bus, which pays drivers hourly rates ranging from $14.05 to $16.75.

He said the larger company had also lessened its chances of a settlement by refusing backpay from the expiry in July of its previous collective agreement.