The car appeared to have run off the road and down a small bank, coming to rest against a fence.
The women's belongings were in the vehicle, Rowbottom said.
He said police were speaking to the woman's family and friends.
"We're trying to track down her movements."
The woman, in her 20s, lived locally in Franklin, Rowbottom said.
It's understood the woman had stab wounds.
She was in a critical condition when she was airlifted to hospital.
An Auckland District Health Board spokeswoman said the woman is now in stable condition.
One man working on a nearby property nearby saw the Auckland Westpac Rescue Helicopter in the paddock. He said paramedics were attending to a person about 150m from the bank.
Several nearby residents were peering over the fence, watching the scene unfold.
Inspector Mark Rowbottom said police were seeking sightings of the woman's station wagon, registration FDP494.
"We'd like to hear from anyone who may have seen this car between 6pm last night and 6am this morning, either in the Waiuku area or in the surrounding South Auckland areas."
"Our investigation is still in its early stages and we are also trying to piece together the circumstances of how she came to be in the paddock and how she sustained her injuries."
Forensic staff were looking at the car when the Herald arrived at the scene. They were also searching the paddock.
Police said the incident was not linked to a search for a man in the same town last night.
