The Boxing Day crash near Porters Pass at 12.50pm was caught on video by a following driver.
The clip shows a silver vehicle travelling along the road before swerving suddenly into the oncoming lane and then back to the left and down a bank.
Matthew Kelly, who caught the crash on his dash camera while travelling with his family, told 1News the vehicle appeared to have clipped the gravel and the driver lost control.
"They overcorrected. They saw oncoming traffic to the left and went off the bank," Kelly told 1News.
"The bank was quite a shallow downhill. (We) got down there quite quickly to check that everyone was OK, but the car was damaged."
The two occupants of the vehicle managed to get out and waited for it to be a towed out.
"Just the sheer fact that we didn't know what was over the bank because it's a rocky area, and we just absolutely thought it would be a lot worse than it was," Kelly told 1News.
A police spokesperson said there were no reports of injuries after the crash.
