Traffic delays are expected on one of Adelaide's busiest roads for the next six weeks after rain damaged concrete installed as part of the $620 million Darlington Upgrade project to turn part of Main South Road into a motorway.

Key points: Heavy rain fell for the first time this year in Adelaide last week

Heavy rain fell for the first time this year in Adelaide last week It has damaged a wall at the Darlington Upgrade project in the city's south

It has damaged a wall at the Darlington Upgrade project in the city's south Traffic will be restricted in the area while the damage is fixed

South Australia's Department of Planning, Transport and Infrastructure (DPTI) said only three southbound lanes of Main South Road at Bedford Park between Sturt Road and Flinders Drive would be open during peak hours, and two during interpeak periods, while the problem was fixed.

Water got behind a wall in the new lowered motorway in Adelaide's southern suburbs, causing spray-on concrete, known as "shotcrete", to crumble.

Transport Minister Stephan Knoll said geotechnical experts would test "the entire length of the motorway to ensure it did not have the same issues".

"It's a pretty ridiculous situation for a new road but, having said that, good that we found the issue now before that lowered motorway is opened," Mr Knoll said.

Workers put a tarp over concrete damage on the motorway ahead of expected rain. ( ABC News: Matthew Smith )

Adelaide had its driest first four months of the year since records began, followed by 24 millimetres of rain in the first four days of May.

Mr Knoll said it was unclear why so little rain would cause such bad damage.

"That's a pretty good question and one I've certainly been asking in the last 48 hours," he told ABC Radio Adelaide.

"… Quite clearly it's not good enough and works are now underway to make sure we fix this on the length and breadth of the motorway."

Mr Knoll said it would make sure taxpayers did not have to foot any extra costs for the remediation.

The Darlington Upgrade is 3.3km long and eliminates five sets of traffic lights. ( Supplied: Department for Planning, Transport and Infrastructure )

Project delayed already

The Darlington Upgrade was originally due to be completed in 2018 but is now expected to be finished by the end of this year.

Since the start of the year, both the Liberal and Labor parties have promised billions of dollars to complete the North-South Corridor project to turn the rest of South Road from Tonsley to the River Torrens into a motorway.

The area is in the state's most marginal seat of Boothby.

A bridge is moved into place for the Darlington Upgrade. ( ABC Radio Adelaide: Spence Denny )

DPTI spokesman Paul Kermode said the concrete damage would not delay the Darlington Upgrade project's completion.

"Our project team is working through that at the moment," he said.

"Obviously there's some more rain, some quite significant rain forecast for this week, so in the short term we've covered up that section to make sure the exposed soil there doesn't cop any more rain in the coming days," he said.

Tarps covering the damage were removed by the time it started raining at midday.

Mr Kermode said the damage appeared to be "localised to that one section of wall".

"But obviously we'll continue our investigations both into the cause of this specific issue and also the broader worksite," he said.

Opposition transport spokesman Tom Koutsantonis called for an independent audit of the project.

"How could it have been allowed to happen, why was concrete poured or why was the structure build that can't cope with a weather event?" he said.

"I have got some news for the Minister: it rains regularly in Adelaide, despite what is going on now with the drought.

"I have got more news for him as well, sometimes it rains and you have vehicles go through it."

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The State Government announced last week that the associated extension of the Tonsley train line across Main South Road to the Flinders Medical Centre would not be complete until next year and would cost $40 million more than expected.

The bikeway connected to the Glenelg tram's bridge over South Road at Black Forest was closed throughout 2017 after pieces of concrete fell from it.

Contaminated cement was used during construction work on the Darlington Upgrade project in 2017.