"The momentum of the car — it was travelling at road speed — and it bounced off whatever rocks hit it and carried on down the road a bit."
The victim's sister, who was driving, was out of the car when emergency staff arrived, Mr Williams said. "She was taken to hospital in Te Kuiti with some minor cuts and probably a bit of shock."
Her sister died at the scene.
The New Zealand Transport Agency, which has offered its condolences to the family and friends of the woman who died, said SH3 was monitored regularly for rock fall activity because the topography made it prone to instability.
Mr Williams said the incident was the fourth firefighters from Piopio had attended involving rock falls on that part of the road. None had previously struck a vehicle, he said.