Emergency services received reports of the single-vehicle crash on Lee Steam-Outram Rd (State Highway 87) about 8am.
A Fire and Emergency New Zealand spokeswoman said crews freed one person trapped in the vehicle, which had "rolled into a paddock".
A Hato Hone St John spokesman said one patient was taken to Dunedin Hospital in a "critical condition".
Graeme Gough, whose vehicle was "about 30 seconds from" the incident, said the aftermath was "quite chilling".
"Three boys jumped out of this truck and were helping as well.
"The woman in the vehicle wanted her seatbelt cut off, but nobody would cut it off.
"They didn’t want to drop her upside down because that’s what would have happened."
Mr Gough said the car must have flipped "at least four times" before landing in the paddock.
"You kind of get back in your car and think ‘that was close’.
"I was quite shaken up.
"If I’d been ... another 30 seconds coming up the road, they would have come across in front of me."
Mr Gough said the emergency response team was "very professional".
The highway was closed for a time to allow for police investigations.
At 3.35pm NZ Transport Agency Waka Kotahi advised the road had reopened.
Another crash closed State Highway 85 at Hills Creek near the intersection of St Bathans Loop early yesterday afternoon.
