The woman was taken to Christchurch Hospital with serious facial injuries and multiple fractures.
"The victim has very serious injuries and it is lucky that she wasn't more seriously injured," Mr Phillips said.
"She was discharged from Christchurch Hospital yesterday and is due to have surgery later this month."
Police are now looking for the three people who attacked the pair.
They are described as a 1.75m tall, solidly built Samoan man, aged about 22-25, wearing a red trucker style cap and a white T-shirt or singlet; a Maori woman, aged about 20-25, of large build; and a young Maori woman, aged about 17-18, of slight build and wearing a black jacket with a fluffy hood.
Anyone who witnessed the incident, can identify any of the people described, or saw the vehicle is asked to call Canterbury Police on 03 363 7400, option 1.