Emergency services were called to Princes Highway at Suttontown, some 445km from Adelaide, at 12.30pm after reports a Toyota Landcruiser and a Ford Territory had collided head-on.
The three passengers inside the Ford Territory, a woman aged 77-years-old and two men aged 55 and 80, all from Millicent died at the scene, South Australia Police said.
At the time of the crash, there were four occupants in the Toyota Landcruiser, two boys aged 16 and 13, an 11-year-old girl and a 48-year-old woman also from Millicent.
The three children were all taken to the local hospital for non-life threatening injuries, police said.
The 48-year-old woman was transported to Flinders Medical Centre by air. She remains in a serious but stable condition.
On Sunday, police said they arrested the 16-year-old boy who was driving the Landcruiser at the time of the crash.
Officers charged him with dangerous driving and he was bailed out and set to appear in the Mount Gambier Youth Court on February 12.
Police closed the Princes Highway following the crash for several hours to allow for Major Crash Investigations to attend the scene, however it has since been reopened.
Police are appealing to anyone who may have witnessed the crash who has not yet spoken to police is asked to call Crime Stoppers on 1800 333 000, or online.
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