Police say the man, 73, was turning onto Wright Street from High Street in Koroit when his white Toyota Kluger was hit from behind by a small truck around 10.55am.

The man received life-threatening injuries from the impact of the crash and was transported to South West Healthcare before being flown to Royal Melbourne Hospital in a critical condition.

"The Koroit man was attempting to turn right at the High Street, Wright Street intersection when a small rigid truck travelling in the same direction crashed into the back of him," Koroit Sergeant Pat Day said.

"As a result of the collision the gentleman's car continued on for about 200 metres and impacted a gum tree out the front of someone's property on the east side of the road.

"Koroit police attended along with Ambulance Victoria, where he was taken to Warrnambool then later airlifted to the Royal Melbourne Hospital.

"His condition is improving but he has sustained a lot of serious injuries."

The Major Collision Unit are investigating alongside Koroit Police.

The driver of the truck, aged in his early 50s, is cooperating with police and assisting with inquiries.

"We would be interested to speak with anyone who hasn't already come forward who saw the collision or who came across it," Sergeant Day said.

"Anybody who has dash cam footage that captured the collision or part of the collision we would really like to speak to them."

Anyone with information is asked to contact Crime Stoppers on 1800 333 000 or Koroit Police Station on 5565 8202.

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