"We looked up and the car just flew over, right in front of us ... rolling down the bank and on to the rocks."
Ms Signal said the car landed within 2m of her daughter-in-law.
"She was right there, she was screaming and running and we looked up as the car was flying through the air.
"It was so noisy and then dead quiet at the end - it was just hush quiet."
Despite the beach being covered in sharp rocks, Ms Signal said she ran frantically down the beach so she could get cellphone service to call for help.
Meanwhile, her son ran to the car to check on the driver.
Ms Signal said she had no idea who was in the car, but immediately assumed the worst.
"I didn't know whether there were children in the car, or a family - it's just bloody awful."
The driver, a 19-year-old man, was in and out of consciousness, but Ms Signal said he was able to say he was from Devon in England.
Firefighters arrived to extract the man from the vehicle before the Auckland Rescue Helicopter Trust transported the man to Waikato Hospital, where he was last night in a serious but stable condition.
Ms Signal said the firefighters did an amazing job.
She hated to think what may have happened if she and her family had not been there.
"If we hadn't been there - if he hadn't just about run us down and landed on us -- because he had internal injuries, he wouldn't have survived the night."
Ms Signal said although the teenager's car nearly landed on the family, she wanted to visit him in hospital to make sure he was alright.
"He was a cool young boy ... he wasn't speeding or anything ... he must have been goggling at the beautiful bay as he was coming around the corner.
"Obviously he was just looking at the bay and he went straight ahead."
In the meantime, the family would keep to the other side of the beach, Ms Signal said.
- Additional reporting: Rebecca Quilliam
