One of Auckland's biggest road-building projects since the harbour bridge is tipped to spark a commercial boom in one of the city's top shopping areas.

More than 150 people are working on a $215 million replacement of the Newmarket viaduct to safeguard the future of New Zealand's busiest stretch of road.

The viaduct, on the Southern Motorway, is used by more than 160,000 vehicles a day and will become four lanes southbound and three lanes northbound.

The southbound carriageway will later become four lanes from Greenlane to Gillies Ave.

At the heart of the viaduct replacement is a complex procedure involving two new bridges to replace the existing structures. Work began in spring and is due to finish in 2012.

A 700-tonne, 140m-long blue gantry above the motorway is being used to lower segments of the new bridges into place.

The project is a partnership between the New Zealand Transport Agency and Northern Gateway Alliance, which built the toll road at the end of Auckland's Northern Motorway.

The agency's Tommy Parker said work was being staged to keep the motorway at almost full capacity. The speed limit on the flyover has been reduced to 70km/h, causing congestion at peak hours, but Parker said the restriction was necessary for the safety of workers and drivers.

The project has the backing of the Newmarket Business Association, despite its potential impact on the busy suburb.

Association chief executive Cameron Brewer said the agency appeared to learn from mistakes made when the original viaduct was built in the 1960s. "A lot of heritage homes were destroyed and communities cut in half. This time we have noticed the agency putting a lot more into communication."

Brewer said the only problem had been the closure of the St Marks Rd motorway onramp, although one lane will reopen tomorrow. He said the project would allow the shopping area to expand. Some buildings at the southern end of Broadway had been bought by the Government and its agencies for the viaduct replacement to go ahead. "In the last five to 10 years there has been very little development down that end of Broadway, largely because those properties have been sitting on death row," said Brewer.

"This new structure will give businesses a lot of certainty and lead to an urban renewal."

Parker said the project would allow the agency to "play a key role in revitalising the forgotten, southern end of Newmarket".

Better footpaths will include a volcanic-themed walkway from Gillies Ave to Broadway.