Adelaide road closed indefinitely due to falling debris

One of Adelaide's busiest roads has been closed indefinitely, affecting tens of thousands of motorists, because of debris falling from an overpass.

An inner-city section of the north-south corridor South Road was closed to traffic this morning after debris was spotted on the road, having crumbled off an overpass.

Transport Minister Stephen Mullighan says the stretch of road will be closed until further notice, causing a "significant inconvenience".

"This part of South Road is one of the most heavily-used pieces of road infrastructure we've got in metropolitan Adelaide," he said at the scene.

He said somewhere between 30,000 - 45,000 people use the road each day.

"We are talking tens of thousands of South Australians who have been impacted by this," Mr Mullighan said.

The problem overpass is made up of two spans, one for trams and the other for cyclists and pedestrians.

While the tram section was considered safe and trams were still running, the bike overpass had shifted, the minister said.

The problem was first spotted by cyclists who raised the alarm at about 7am.

Bridge experts and structural engineers will inspect the site and determine the best way to return traffic to the road.

"Safety is our first concern," Mr Mullighan said.

"We want to make sure we're not putting any motorists, pedestrians or cyclists in harm's way."

The minister says the government will also get to the bottom of who or what caused the problem, and will "definitely be holding those people to account'.

The overpass was designed and certified by independent experts and constructed about seven years ago.