A 69-year-old Dunedin woman was driving the vehicle when it hit a road barrier and flipped, landing on its roof in the south-bound lane of State Highway 1 just after 3pm, police said.
Two females, aged 17 and 18, were removed from the back seat of the wrecked car and one was flown to Dunedin Hospital in the Otago Regional Rescue Helicopter. Both were later released from hospital.
One of teenagers said her friend's grandmother and grandfather, who was a front-seat passenger, were treated at the scene by St John staff and transported by ambulance to Dunedin Hospital.
She said both were under observation late last night.
Constable Lesley Eason, of Waikouaiti, said it appeared the driver fell asleep at the wheel.
No other vehicles were involved in the crash, which temporarily closed SH1.
Traffic was diverted while the highway was cleared.
A nearby resident, who declined to be named, said he heard the crash before seeing the late-model car "fly through the air".
It was the second accident within a week along the same stretch of road, he said.
On Monday, a car with five occupants flipped and landed on its roof in a paddock near Goodwood.
The 17-year-old Dunedin driver was inexperienced and struggled to control the car in heavy rain.
All five teenage occupants were transported by ambulance to Dunedin Hospital with minor injuries.
The Waikouaiti and Palmerston volunteer fire brigades attended both incidents.
