The woman had six passengers in her sedan, which seated only four passengers.
"The mentality of this person is beyond comprehension," Snr Sgt Dinnissen said.
The passenger in the boot suffered a large cut to the back of the head.
"There is no reason whatsoever that you should be sticking six people in a car when it's fit for four."
The driver was facing charges of careless driving and causing injury, he said.
St John communications adviser Gerard Campbell said the woman in the boot was one of two patients taken to Dunedin Hospital in moderate condition on Monday night.
On Saturday five teenage boys died in a crash in Timaru. Some of them were not wearing seatbelts.
One of the boys was found in the boot of the car.
Aoraki area commander Inspector Dave Gaskin, who attended the Timaru crash, said he did not know what it would take to get the message through to young people.
"Young people do silly things.
"Ninety per cent of the time you get away with it, but sometimes things go bad."
Riding in the boot of a car was inherently dangerous, he said.
Passengers needed to ensure they sat in the correct seats and wore seatbelts while riding in a car, regardless of the situation, he said.
Senior Sergeant Nik Leigh, of Dunedin, said he was amazed to hear about the incident, just days after the tragedy in Timaru.
"A lot of these people think it's not going to happen to them, but it does."
Many people had got away with risky behaviour on the road, he said.
"In my job, we have to pick up the pieces of those who didn't get away with it."
