A Strathalbyn man has sustained life-threatening injuries in a road crash overnight.

Police were called to Nine Mile Road around 20 minutes past midnight after a car travelling towards Milang left the bitumen and ran into a tree.

The 34-year-old man at the wheel was choppered to the Flinders Medical Centre in a critical condition.

Police are still investigating the cause of the crash, and have asked anyone with information that may help to get in touch.

• Crime Stoppers: 1800 333 000 or www.crimestopperssa.com.au.