Michael Baty had been aware Colleen Reid was missing on Tuesday, when he caught a glimpse of something red, which turned out to be her car.
He pulled over, made his way through trees and shrubs and rang emergency services. A police officer was quickly there, he said.
"At first, we couldn’t get into the vehicle, but she was giving us the thumbs-up and all that," Mr Baty said.
He was surprised, because she had not been seen for almost 24 hours.
A family member said afterwards Ms Reid, who lives in Allanton, was well and coherent at Lakes Hospital.
She had been driving to Queenstown Airport to pick up a relative on Monday but failed to arrive.
Mr Baty, who works for construction company Faulks Investments, talked to the Otago Daily Times yesterday about his experience.
He was reluctant to accept much credit, describing a team effort also involving firefighters and police.
"I’d already done a trip to Cromwell and I was on my way back with the next load, slowing down for the Nevis Bluff stop-go," Mr Baty said.
"Out of the corner of my eye I caught a glimpse of something red.
"It was opposite the Oxbow adventure centre. It was just in the bushes in there, on the right.
"I pulled over and ran back and that was the car."
The vehicle was on its side and branches were broken over the top, he said.
Emergency services soon took over.
Police thanked members of the public for their help.
"I happened to be at the right place at the right time," Mr Baty said.
"Me, the cops and the fireys, I’d call all of them the heroes - not just one of us."