Blenheim police say driver inexperience and illegal modifications to a vehicle were the likely reasons for a crash south of Seddon on Sunday afternoon.
Sergeant Barrie Greenall says the 16-year-old driver lost control on a straight section of highway and the Subaru rolled several times.
He says the driver was on a learner's licence and the other occupant of the car, a 32-year-old woman, was lying on the back seat unrestrained when the crash happened. She suffered serious injuries.
Mr Greenall says conditions were dry at the time. The crash is being investigated.