Two 3,000-tonne bridges will be built off the side of the Darlington Upgrade Project in Adelaide and moved into place to avoid traffic disruptions.

Construction on the 180-metre-long bridges has begun between the Southern Expressway and Main South Road, near Marion Road.

The bridges, which will be moved into place over two weekends in October and November, are part of the $620 million North-South Corridor project.

Transport Minister Stephen Mullighan said moving the bridges into place would be an Australian first for a civil construction project.

Transport Minister Stephen Mullighan inspects the construction site where two bridges will be built. ( ABC News: Alina Eacott )

"Today what we are seeing is massive beams starting to come on site from a South Australian company, Bowhill Engineering," Mr Mullighan said.

"They will be constructed into two 3,000-tonne bridges which will be moved into place via those small automated crawling trailers.

"There's a massive reduction in how much we disrupt traffic as we are building these bridges and as we move them into place."

He said nearly 70,000 vehicles a day use the road, the busiest section of South Road.

When complete, the Main South Road bridges will take northbound traffic over the Southern Expressway.

A total of eight bridges will be built for the Darlington Upgrade Project, which is now expected to be completed in 2019.

Concrete pillar worries

Meanwhile, the State Opposition said it was worried about the safety of a pedestrian overpass at the Lonsdale train station in Adelaide's south.

David Pisoni and Dr Kenneth Moxham inspect the Lonsdale overpass. ( ABC News: Alina Eacott )

Opposition transport spokesman David Pisoni said a member of the public raised concerns about cracking in one of the overpass's support columns.

The State Government is currently inspecting about 1,500 bridges around the state following problems with the overpass at the Glandore tram station in January.

Mr Pisoni said the community needed to know the infrastructure was safe.

"We're calling for the audit to be released. If there's an interim report, we'd like to see that report, and we'd like to see what the audit response was to this particular bridge," he said.

Engineer Dr Kenneth Moxham, who accompanied Mr Pisoni at the train station on Sunday, said he was concerned there could be a "very serious problem" given the cracking.

"It is difficult to be certain what caused it," he said.

"It could be cosmetic and it could be particularly serious and it is the fact that it could be a very serious matter that worries me. That has to be eliminated.

"It is possible, it is not probable, but it is possible the entire bridge could fall down."

Mr Mullighan said the overpass had been inspected as part of the ongoing audit and engineers "don't anticipate that there is significant structural damage or that there is any risk to the public from either using the bridge or being in the vicinity of the bridge".

"It is likely to require some repairs, certainly at least to the facings where that concrete cracking has emerged," he said.