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Auckland road police are bracing for severe traffic disruption over the Anzac holiday weekend if bad weather holds up a big repair job on the Northern Motorway.

Transit NZ says it urgently needs to reseal 3km of the motorway between Oteha Valley Rd in Albany and Silverdale in Rodney District before winter to fix cracks likely to cause widespread surface failures if left unfilled.

The agency intends beginning the $5 million-plus project as early as tomorrow night by closing one of the motorway's two southbound lanes.

It does not expect the job to be finished until Monday, May 5.

But although it has timed the work for the two weeks of school holidays which start today, because it expects lower overall traffic volumes, police are concerned about the possibility of bumper-to-bumper queues over Anzac weekend.

Transit has promised to do its utmost to ensure both southbound lanes reopen by noon on April 27, the Sunday after Anzac Day, to make them available to holidaymakers returning to Auckland.

And it says it will not start work on the northbound carriageway until 9 o'clock that night.

Southbound traffic will be reduced to one lane with a 50km/h temporary speed limit for the first week of the project.

Northbound drivers will face the same constraints in the second week.

Drivers are being urged to use State Highway 17 through Albany or East Coast Rd as alternative routes.

But the head of the Waitemata police highway patrol, Senior Sergeant Bill Russell, acknowledged yesterday there could be no guarantee the southbound carriageway would reopen in time.

"It depends on the weather - if it rains, their sealing has to stop."

Transit's northern operations manager, Joseph Flanagan, said his agency had carefully assessed traffic projections before deciding to time the project for the school holidays.

"If we did it during normal weeks there would have been too many delays - there would have been very severe traffic congestion."

He was confident the benefit of reduced peak traffic volumes over the school holidays would outweigh potential short-term difficulties over the Anzac weekend.

Transit was treating noon on the Sunday after Anzac Day as its absolute deadline to reopen the full southbound carriageway, and hoped to do so with hours to spare.