"By the time she was put in the ambulance she was critical.
"It appears she has head injuries."
Police at the scene said speed was not a factor, but the woman had to have turned the wheel to her right in order to leave the road.
"She has come down here and for some reason she has turned the wheel," Hastings Senior Constable Tim Rowe said. "Why that is we do not know.
"There is no evidence to show she braked on the way down".
After hitting a grass verge, the car continued down a grass slope, hit a tree on the passenger side before falling bonnet first over a steep bank and landing in a ditch.
The cause of the accident was still under investigation.
"We will run through any medical reasons, check that the brakes are functioning, then we investigate anything else that could have affected her - drugs, alcohol or fatigue," Mr Rowe said.
In April last year, a group of four teenagers in a car were lucky to survive after plunging 150 metres down a bank on the first left-hand downhill bend on Te Mata Peak. They received only minor injuries.
Last October, Taradale pensioner Mervyn Carr, 72, plunged to his death off Te Mata Peak after failing to take a corner in heavy fog. A coroner's report into the incident suggested that safety barriers be installed on the summit road.