Auckland's $230 million duplicate Mangere Bridge is set for completion before the end of this year, in ample time to greet visitors arriving for the 2011 Rugby World Cup.

Accelerated progress on building the bridge and widening 4km of its motorway approaches means the project will be completed well ahead of its target date of March 2011, although the Transport Agency has yet to confirm that.

Onehunga community sources believe it could be ready as early as October, a far cry from when the former Transit NZ fretted in the planning consent stages that it might not be available to ease traffic congestion for the tens of thousands of visitors expected through Auckland Airport for the rugby tournament.

All the Transport Agency is saying for now is that it looks "increasingly likely" that the project, which a Fletcher-led construction alliance began 21 months ago, will be completed ahead of schedule.

But the project is more than 70 per cent complete, with most of the motorway widening finished between Queenstown Rd in Hillsborough and Walmsley Rd in Mangere, and more than half the road decking in place across the 640m bridge's seven piers.

That is a stellar performance compared with the existing Mangere Bridge, which opened in 1983 after taking eight years to build amid interminable labour demarcation rows and a redundancy pay dispute which halted construction for 2 years.

The duplication project has the advantage of heavier cranes, and a low temporary surface bridge across the harbour for easier construction access.

Today's workers even appear to have accepted a ban on fishing off the surface structure during meal breaks, to protect the public image of the project in a community which one industry source said remained "emotionally scarred" from the earlier bridge saga.

The project is approaching a sensitive phase in which the bridge decking across the Manukau Harbour must be connected above busy Onehunga Harbour Rd, at the northern end, to three new southbound motorway traffic lanes and a bus shoulder lane.

The road will be shut to traffic between 11pm and 5am for five nights a week between about January 27 and early March, as a 50 metre gap is closed overhead between the bridge and the motorway.

That will require 14 concrete segments, each weighing about 55 tonnes, to be lifted into place above the road by a crane - creating an imposing cantilevered structure before it can be connected to an abutment holding up the motorway lanes.

The lifts will not be as taxing as when two of the country's largest cranes worked in unison to raise two 245-tonne blocks of concrete on to piers at the Onehunga end of the bridge last month.

But Transport Agency regional director Wayne McDonald said they would be undertaken at night for safety reasons while Onehunga Harbour Rd was closed, and the start of the operation would signal that the end of the project was in sight.

The harbour road is used by northbound traffic leaving or joining the existing motorway, which will have to use detour routes through local streets on nights when it closes, between Sunday and Thursday for about six weeks.

Construction may also start towards the end of that period on a neighbouring footbridge across the same road, to carry pedestrians and cyclists using the older Mangere Bridge, which was built in 1914 and was closed to motor traffic when the existing motorway crossing opened.

Mr McDonald said special arrangements were being made to ensure access was maintained to the western end of the harbour road, including to Onehunga's port.

The new bridge will provide southbound traffic with four lanes, leaving the existing motorway crossing just for northbound traffic.