When police arrived they couldn't reach the car because of the swollen stream, he said.
"Then the fire truck showed up ... by then there was nothing anyone could do really."
Mr Jenness said he did not know there was anyone in the car at the time he called emergency services. "I thought maybe people had parked up there and gone for a walk along the riverbank.
"I got a call from police later that day, and they said that there was an 80-year-old man floating around.
"That was a bit stink. I felt like I could have done something but it was probably too late anyway."
Dr Hill was a doctor and physiologist who had taught at Auckland University's medical school. His family described as a much loved man who would be missed by many.
"He was a climber, a traveller, had an active and inquiring mind and a life-long love of learning new things. He took great delight in walking the hills of Wellington with his beloved dog, enjoyed his membership of the Alpine Club, and shortly after his 80th birthday completed a solo tramp of the Kepler Track."
Dr Hill was born in India and brought up in Ireland, but thought of New Zealand as his home, his family said. "He was a very loving and much loved husband, father and grandfather, brother and uncle, and will be sorely missed by all who knew him.
"Our family wishes to express our gratitude to all those who assisted in the search, and for the kindness and professionalism of the police."
Hutt Valley Area Commander Sean Hansen said it was a tragic incident.
"Our thoughts are with Dr Hill's family at this difficult time," Mr Hansen said.
A man, who did not wish to be named, was at the scene of yesterday's incident.
He said Dr Hill's family were frantically searching for him as floodwaters in the area rose.
Dr Hill had been driving to meet his family for lunch nearby, and when he failed to arrive, his family began searching for him, he said.
The man said he didn't get any sleep last night.
"You read about this sort of stuff happening ... but actually being there and seeing it all happen is different."
Dr Hill's death has been referred to the coroner.
