A woman is extremely lucky to be alive after her car drove off a rock wall of a car park into a harbour and immediately began sinking at a frightening speed.

The petrified woman was squashed up against the rear window as the front end of the BMW was rapidly submerging into the Waitemata Harbour in Auckland at 3pm on Tuesday.

Fortunately witnesses called emergency services as quick-thinking policemen arrived and sprung straight into action by jumping into the water.

The officers and a bystander couldn't open the doors so one officer attempted to smash the window with a baton with no luck.

Once it became clear that it was not going to penetrate through the glass another officer was handed a rock and managed to shatter the rear window.

The relieved woman was then pulled to safety by the two heroic policemen.

The two officers who saved the woman's life - Paul Watts and Simon Russell - have described the dangerous rescue, and revealed they believe she could have been less than a minute away from drowning.

'When we entered the water, I was talking to one of the (members of the public) who was trying to balance the vehicle and he said he was looking for a rock to try and smash the window,' Mr Russell told the New Zealand Herald.

He said once his baton failed he successfully broke through the window with a rock, but by this time the car was sinking at a rapid rate.

'At that time the car actually started to move very fast into the water. I was trying to hold the car, slowing it down from entering the water,' Mr Russell said.

Mr Watts and Mr Russell then managed to pull the woman from the car to safety, but said they got her out just in time.

'It was pretty close, probably 30 or 40 seconds after we managed to get the female out of the car, the car was already slipping further into the water,' Mr Watts said.

'I'd say she'd probably only had maximum probably another minute, minute-and-a-half if she hadn't got out,' he told the NZ Herald.

Mr Watts described the dramatic rescue as 'pretty much part of a routine job being a police officer' in another interview, saying the whole operation only took about 40 seconds. The car was fully submerged just a minute after the woman was freed from it.