The campervan, which had been travelling south on SH10 and towing a boat, had damage to the front right corner. The male driver, also a sole occupant, was unhurt.
Sergeant Kevin Milne, of Mid North police, said the seriously injured woman had been flown to Whangārei Hospital.
The Serious Crash Unit had carried out an investigation to determine the cause. Neither speed nor alcohol was thought to have contributed.
Kerikeri fire brigade station officer Andy Hamberger said the volunteer firefighters had unhitched the boat, pulled the campervan off the car and secured it to another vehicle with a strop, then cut off the driver's door and the roof to extract the patient.