Marcus Reid, of Wanaka, said he was driving along Portobello Rd towards Dunedin about 11am when he came across the Honda sedan sitting at a near-45deg angle on the seawall west of Macandrew Bay.
He noticed the woman driver, who he thought was in her 20s, still sitting inside the car, so stopped and approached on foot.
After checking the vehicle was stable, he carefully opened the front-passenger door.
The woman, who was shaken but not injured, was then able to carefully climb out of the vehicle to safety.
Mr Reid said the woman was on her cellphone when he approached the car.
She had called emergency services, but was too scared to shift in case the car, which appeared to be held in place only by a 50cm-long rock halfway down the wall, rolled over.
He understood the woman was driving slowly in a hail shower when her car just slid off the road, Mr Reid said.
"It was pretty amazing it didn't just roll down. She could have been really hurt."
The car, which received minor damage, was removed by a tow truck about 20 minutes later.
