Despite rescue attempts the driver could not be saved.
Auckland student Jack Cameron, 19, was on a run along Westhaven Marina and had stopped to sit down by the water.
At 7.15pm a car, driving about 30km/h, swung around the nearby roundabout and launched past him into the water.
"A car came around the roundabout and just went straight into the drink," Cameron said.
It was barely 5m from where he sat.
"I looked over and the door opened and I thought they were getting out.
"Then another car arrived and so I was looking at that, then a woman hopped out and said 'Why aren't you in there?'
"So then I looked back and the person wasn't out."
Men on a nearby fishing boat smashed the back window of the car but the vehicle was almost submerged and rapidly went under.
"These guys [fishermen] came over and smashed the back window and that's when I jumped in.
"By the time I got there the car was well gone. I couldn't do anything."
Cameron could see only one person in the silver car when it hit the water, just next to the Harbour Bridge.
He said the water was "cold and murky" when he dived in. "I couldn't see anything," he said.
He was still in wet clothes and wrapped in a towel but he and the men in the boat stayed at the scene until the car was pulled out of the water
"I just want to see the body returned to the family."
