Disruption to commuter services would be unavoidable, but he hoped buses would be able to clear backlogs reasonably quickly after 4.30pm.
But the unions say NZ Bus could have spared itself and its passengers the trouble by observing an agreement for authorised stop-work meetings to be held at off-peak times at the company's expense.
Mr McMahon earlier denied the agreement required the company to sponsor meetings unlikely to lead to drivers accepting a pay offer, but First Union transport secretary Karl Andersen said it had gone back on the deal.
"We've got a signed agreement with them and it says meetings will be held during certain times at their expense, and they've reneged on it," he said.
"So if there's any unnecessary disruption, the fault lays directly and squarely with them."
Despite the company's prediction of disruption, Auckland Tramways Union president Gary Froggatt said he expected drivers would arrive back at their depots ready to resume normal services from about 3.30pm to 4pm.