Three Mid North youths, all thought to be about 18 years old, were in the car. Two were able to get out of the wreckage by themselves but firefighters had to extricate the backseat passenger.
Both passengers were initially in a critical condition with multiple injuries.
Their condition had improved by the time they reached hospital and both are expected to survive. The driver's condition was initially serious but improved to moderate.
One patient was flown directly from the crash scene to Whangārei Hospital by the Northland Rescue Helicopter but the weather had closed in by the time a second helicopter arrived from Auckland.
The other passenger was transported by St John Ambulance to Rawene Hospital, where the chopper was able to land using instruments.
Volunteer firefighters provided life-preserving first aid and stopped traffic, Bristow said.
The highway remained closed until 10.30pm while police carried out an investigation.
Senior Constable Warren Bunn, of the police Serious Crash Unit, said the vehicle was travelling west in wet conditions when it slid off the road at a right-hand bend and hit the container, coming to rest upside-down.
There was a strong smell of petrol at the scene — either the tank ruptured or the cap came off when the car was on its roof — so the occupants were "very lucky" it didn't ignite.
At least one of the occupants was not wearing a seatbelt because he was found pressed up against a pillar.
"His injuries didn't have to be that serious. Drive to the conditions and always wear a seatbelt," Bunn said.
Blood samples had been taken for alcohol analysis.
