Witnesses saw a car's occupants climbing on to its roof and bonnet and a passenger urinating out of a window before it crashed on Friday.
Police said the car's four teenage occupants were very lucky to not have sustained serious injuries.
Acting Inspector Phil Dean said the blue Subaru Impreza left Hanmer Springs about 8.30pm last Friday travelling towards Christchurch along State Highway 7 and SH1 before it crashed on SH1 at Kaiapoi.
"We received multiple complaints from members of the public of the car overtaking at speed and some of the occupants climbing over the bonnet and roof of the vehicle while it was travelling along the State Highway."
Police said witnesses reported the front-seat passenger was standing waist-high and urinating out of the side window before they saw him sit back down and the driver lean her body out of the window shortly before the crash about 10.50pm.
The car is then understood to have veered across the southbound lanes and become airborne when it collided with a centre barrier, landing in the middle of the northbound lanes.
"All of the occupants were extremely lucky not to have seriously hurt themselves or anyone else when they crashed. Police are investigating claims that the front-seat passenger was steering the car while the driver was leaning out of the window," Mr Dean said.
The 18-year-old woman driving and 17-year-old male rear-seat passenger were taken to Christchurch Hospital by ambulance with moderate injuries to their head and face.
The 19-year-old woman passenger was uninjured and the 17-year-old front-seat passenger had minor injuries to his right shoulder.
Police investigations into the crash are continuing. The driver and several of the occupants of the car received infringement notices in relation to their unsafe behaviour while the car was moving.
Police said it was likely charges would also be laid against the driver for her manner of driving.
"The roads in Canterbury are as safe as those using them and in this instance the driver and some of the occupants in this car put not only themselves but other road users at risk with their unacceptable behaviour," Mr Dean said.
"On behalf of Canterbury police I would like to thank the witnesses to this incident who have provided valuable information to the investigation into the cause of the crash. It is unacceptable for a minority of road users to put others at risk with their unsafe behaviour and erratic driving."