Chris Mahon tried to stop it with his bare hands - but in the end he could only watch in horror as his expensive Land Rover Discovery rolled backwards into a dam.
Mahon, a retired director of the Irish Law Society, said he had run down his steep driveway after the runaway vehicle and tried to stop it.
But it was too heavy and he cut his hands in the attempt.
His 4x4 plunged into the tranquil Sonstraal Dam opposite his house in Bokmakierie Street, Durbanville.
Mahon said he had gone out on an errand, but returned home to fetch a pair of glasses he had forgotten.
"I had just climbed out of the car when it started rolling backwards," he said on Tuesday.
"I tried to stop it but it was too heavy and just picked up speed.
"It cleared the gates at the bottom of my driveway, careened over the street and down a grass embankment towards the water."
The peace of Sonstraal Dam, home to hundreds of ducks and geese, was shattered with a splash and the scattering of birds in all directions when the Land Rover plunged into the water and sank.
When the turbulence subsided, the curious birds returned to investigate the strange visitor.
Mahon's car was so deep underwater it was not visible from the bank.
The first police reports raised fears that people were trapped in the Land Rover.
Police divers reached the scene in minutes, found the submerged vehicle and searched it as a matter of course.
They recovered the driver's side door, which had been ripped off, and the remote control for his garage door.
Accident recovery staff later winched the waterlogged car out of the dam.
Mahon could not say why his car had started rolling.