The car the man was driving crashed into a water race on State Highway 83 between Kurow and the Kurow Cemetery about 10.30pm.
Constable Craig Bennett, of Kurow, said the man was driving while tired and struck a large boulder on the roadside.
The car left the road and came to a rest the right way up in an irrigation race.
He said the water race was about 1.5m below road level and was usually flowing with water at a height that would have been over than the vehicle's windshield.
The man was able to get out of the car and stop a passing motorist. He was taken by ambulance to Oamaru Hospital with minor injuries and later released.
Police inquiries were continuing, but Const Bennett said the crash was a reminder to motorists not to drive while fatigued.
