They fell from a bluff about 30m above the road, Mr Williams said.
`"Right at the base of the bluff, there's quite a steep angle. The rocks hit the bottom and bounced out onto the road. It struck the car on the side.''
"The momentum of the car - it was travelling at road speed - and it sort of just bounced off whatever rocks hit it and carried on down the road a bit.''
Ms Thompson'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 incident was the fourth firefighters from Piopio had attended involving rock falls on that part of the road, he said.
None had previously involved vehicles being struck, he said.
The dry weather could have contributed to instability in the area.
"It's a 50km stretch of road, and it's in a gorge and its [rockfalls] just one of the hazards that are there. It's well sign-posted. It was unfortunate. She was unlucky.''
Senior Sergeant Pete Van De Wetering advised SH3 was down to one lane.
Traffic management was in place and motorists should take extra care when using the affected stretch of road, he said.
With the Beach Hop event in Whangamata and the Balloons Over Waikato festival in Hamilton, roads in the Waikato region were busy, Mr Van De Wetering said.
People should plan ahead and allow plenty of time to reach their destination, he said.
