Mrs Ballantyne had to swerve to avoid hitting the car as it crossed into the southbound lane in front of her.
"He had lost control when we saw him. He ploughed into the bank and flipped over," she said. "My daughter was yelling out, 'Mum'. I just looked into my rear vision mirror and saw no one was coming so I just swerved. You don't think, I just did it."
Mrs Ballantyne said it was lucky there were no other cars nearby at the time or she too would have crashed. She pulled over and ran back to stop an oncoming truck while her daughter phoned emergency services.
"He pulled over and hopped out of his truck. I called out to him to go and help the guy and I continued to direct traffic."
It was while she was controlling traffic that the situation really sunk in.
"You just go into shock," Mrs Ballantyne said.
Tauranga South strategic traffic unit acting Sergeant Mark Duytshoff said the elderly driver was heading towards Katikati when the accident occurred.
"He's lost control on the corner and ended up on the wrong side of the road, where he's hit a bank and rolled the vehicle."
The accident held up traffic for more than an hour as one lane was closed.
Western Bay of Plenty road policing manager Senior Sergeant Ian Campion said it was still not yet clear what caused the crash.
"We basically know what happened, it's just finding out why." He was unsure if there had been any other crashes on that particular corner.
"I know that State Highway 2 between Katikati and Bethlehem does cause us some concern and has a reasonably high crash rate, but it does carry a high volume of traffic."
A Tauranga Hospital spokesman said the man was in a stable condition with non-life threatening injuries and would remain under observation.
