Senior Constable Dan Hales said the Australian woman, who was a passenger in the rental car, was airlifted to hospital in a serious condition.
"The female has sustained quite severe injuries and her husband was comforting her in hospital. She had suspected neck, rib and pelvis injuries and he had minor chest injuries," Mr Hales said.
Witnesses had reported seeing one of the occupants of the stolen Subaru being picked up by a flat-back truck and asked anyone who had given a hitchhiker a lift to contact them.
"Also, to these guys, to leave the scene prior to police arriving in an injury accident - we'd request the driver and the passenger came forward," Mr Hales said.
The vehicle had been stolen between 10.30pm on Thursday and 5.20am yesterday from a Wolfe St address in Whangarei. The couple had just landed in New Zealand from Australia and were heading to Kerikeri to visit family. The injured woman's husband is a Kiwi.
Mr Hales hoped to be speaking to the couple in the next two days.
The male was discharged from hospital and the female was in a stable condition yesterday.
Mr Hales said the stolen Subaru had been forensically investigated.
Anyone who saw a blue Subaru Impreza with gold mag wheels some time between 10.30pm and 5.20am yesterday or anyone who picked up a hitchhiker around the time of the accident is asked to contact Dan Hales on 4304500 or, to remain anonymous, Crimestoppers on 0800 555 111.