Major Australian highway closed after horror crash - with three people rushed to hospital in a critical condition

Three people are fighting for their lives after the car they were in collided head-on with a truck on a major highway.

Emergency services were called to the Riddoch Highway just south of Naracoorte, near the border of South Australia and Victoria, about 9.30am on Thursday after reports of serious collision.

Three passengers in the car sustained 'life-threatening injuries' and were rushed to hospital, South Australia Police said in a statement.

The truck driver was taken to hospital for treatment for minor injuries.

Police have closed the highway between Edenhope and Wrattonbully Roads as investigators and Heavy Vehicle Enforcement Section officers travel to the scene.

Diversions have been set up around the highway which is expected to remain restricted to the public for 'several more hours'.

Commuters have been urged to avoid the area.

More to come.