"My natural reaction would have been to launch myself with a dinghy into the water," the man said.
"I saw it go in [and] I watched it sink. I maintained my position to tell police where it had submerged.
"If I could have jumped in the water and freed the girl I would have.
"[But] I was the only person able to talk to [emergency services] about [what was] going down."
Police this afternoon confirmed that the body of a woman had been recovered from inside the vehicle.
The police dive squad arrived at the scene this morning to recover the woman's body and she was believed to have been the only occupant of the car.
Her body was taken to Auckland Hospital for a post-mortem examination and the "process of identification and notification of the next of kin is ongoing".
Police said no one else was being sought over the incident.