Two people have died on South Australian roads this morning.

Police say at about 6.45am this morning a Mawson Lakes man, 37, was driving along Highway One, six kilometres north of Lochiel in the state's mid-north, when he lost control of his car.

His car left the road and rolled.

He was the only person in the car and died at the scene.

Police and emergency services were at the scene this morning and were diverting south bound traffic around the crash site.

Drivers have been asked to be patient.

Man dies at Mount Gambier

Later this morning, a Mount Gambier man, 89, was on a gopher crossing the Jubilee Highway at West Mount Gambier, when he was hit by a truck.

He suffered a cardiac arrest at the scene and was rushed to hospital, where he later died.

Major Crash Investigators have travelled to Mount Gambier to investigate the cause of the accident.

Police say the Jubilee Highway will be closed for several hours, between Wehl St and Suttontown Rd.