Aucklanders are being lauded for staying off the roads at the weekend during a mammoth traffic management operation which transport officials believe bodes well for next year's Rugby World Cup.

Traffic on major arterial routes was half normal levels during New Zealand's largest motorway closure - of the Newmarket viaduct's southbound lanes - from just before 6pm on Saturday.

That came as a relief to the Transport Agency, which organised the closure, and the Auckland Regional Transport Authority, which laid on more trains and ferries.

The authority regards the public response as an encouraging indication of the region's ability to cope with challenges such as next year's rugby tournament.

By noon yesterday, the viaduct was open and carrying traffic along a new 690m southbound carriageway, 17 hours before a deadline of 5am today.

Business was brisk for Father's Day in Newmarket, after a detour through the southern end of Broadway caused little of the disruption feared by the area's business association.

The Transport Agency also blocked three city on-ramps to the Southern Motorway during the closure to discourage traffic.

By 11.30am yesterday there was still a trickle of vehicles on the approach to Newmarket's Gillies Ave off-ramp, from where drivers were diverted down Mortimer Pass,

The closure, needed to switch traffic from a carriageway which will be demolished in the next stage of a $215 million viaduct replacement project, will be followed in 2012 by a similar exercise affecting northbound traffic.

It was preceded by weeks of warnings from the Transport Agency and its construction partners.

A $100,000 advertising budget was augmented by email campaigns and messages on social networking internet sites.

Agency traffic management unit chief Mark Walker yesterday defended the scale of the warnings.

He said that if drivers had not heeded them, local roads would have been unable to cope with anything like the 60,000 southbound vehicles using the viaduct on a normal Sunday.

Provision was needed for a full 36-hour closure in case of hitches in the complicated task of moving an 800-tonne lifting gantry between the old and new southbound lanes.

"I wouldn't have liked to guess they could have done it any faster."

Mr Walker acknowledged that the agency would face another big challenge persuading drivers to stay off the roads early in 2012, when it switched traffic to a new northbound viaduct carriageway.

But that would be somewhat easier to plan because of a greater choice of northbound off-ramps through which to divert traffic to detour routes.

Despite the early reopening of the southbound motorway lanes, the regional transport authority honoured its promise to pay for free train trips all day yesterday, as well as from 5pm on Saturday.

The authority also approved a plan for Veolia Transport to run longer and more trains, which communications manager Sharon Hunter said carried six times more people yesterday than during a normal Sunday.

That was the equivalent of about 30,000 passenger trips.

She not believe a motorway closure would have been possible with such little disruption 10 years ago, when rail services were far less frequent and reliable.

Newmarket Business Association chief Cameron Brewer, who feared gridlock through the suburb would stifle Father's Day shopping, congratulated the viaduct builders on running such an effective communications strategy and reopening the motorway so quickly.

"All is forgiven," he said, adding that any loss in sales his members might have suffered was nothing "compared with what our friends in Christchurch are going through".