Construction work is starting on the first major new interchange to be built on the South Eastern Freeway in the Adelaide Hills in almost two decades.

The interchange at Bald Hills Road at Mount Barker is expected to improve commuting times and road safety for people travelling to the eastern side of Mount Barker and cut traffic congestion in the Hills community.

Federal Assistant Infrastructure Minister Jamie Briggs and South Australian Transport Minister Stephen Mullighan symbolically turned the first sod for the road project, which will create on and off ramps in both freeway directions and a roundabout at the junction of Bald Hills Road and the Old Princes Highway.

The Federal and State Governments and local council are spending $27 million on the interchange, which is due for completion by the middle of next year.

More planning for South Road work

Mr Briggs and Mr Mullighan hinted to reporters that a further funding announcement was imminent on the South Road project through Adelaide.

They said they had been in discussions about the project.

Mr Briggs said it would make sense to prioritise the Northern Connector project, a proposal to link the Northern Expressway with the South Road Superway, given that other parts of South Road would be congested with construction work in the next few years.

An aerial view of the planned Bald Hills interchange. ( Supplied: Dept of Transport )

"The Prime Minister made a commitment to deliver an upgraded South Road corridor within 10 years, that's what we're working towards and that will obviously involve substantial new investments in South Road in the next little while, and we hope to have something to say ... when we come to an arrangement," he said.

The current South Road projects will upgrade sections between the River Torrens and Torrens Road, and further south at Darlington, removing traffic lights and putting South Road under some other major roads.

The long-term plan is to have a road corridor through Adelaide with as few stops as possible.

Earlier projects put South Road under Anzac Highway, built the elevated Superway next to the existing South Road in the northern suburbs and duplicated the Southern Expressway in Adelaide's south.