The highway was closed for nearly three hours as emergency services worked to free the woman and clear the scene.
The woman was trapped in the wreckage of her car for most of that time before firefighters were able to cut her free and the TECT Trustpower Rescue Helicopter flew her to hospital.
Traffic diversions were put in place down Old Coach Rd for most vehicles, except heavy trucks.
Western Bay of Plenty police Senior Sergeant Shannon Clifford said it was too early to indicate the cause of the crash which was being investigated by the Serious Crash Unit.
"Police are seeking any witnesses to the crash, and ask that anyone with information contact Tauranga police on (07) 577 4300 or alternatively anonymously on Crimesstoppers 0800 555 111."
Maketu fire chief Shane Beech said the woman had been "in a bad way" and "in and out of consciousness".
Mr Beech said both cars were write-offs.
"The two cars were jammed between different barriers on the bridge," he said.
Mr Beech said that part of the highway was known for crashes.
"It was identified a few years ago as a potential black spot on that highway.
"Touch wood we haven't been for anything serious for a while [with the exception of this crash]. In years past there have been many crashes there."
In 2010, five children including a baby were mistaken for animals lying across the road after a "horror" crash when the car they were travelling in collided with a barrier.
In 2012 a truck and trailer unit carrying kiwifruit rolled. The driver was not seriously injured but the road was closed for several hours.
