The witness - who did not wish to be named - said the noise of the impact woke him and his wife, who are tenants in the house where the crash happened just below their bedroom window.
"I ran outside. At the same time the next door neighbours, two guys came out of that place," he said.
"They were on his side, and I was on the passenger side and I opened the [car] door to see what was going on."
"He was knocked out, and making groaning noises.
"There was a bit of timber from the fence that went through his window, and he was sort of pinned by it over the top of his head, and it went out the windscreen."
The three men tried to hold him still while attempting to remove the piece of timber that was holding him in place.
A woman phoned emergency services, who told them not to try to move the injured driver.
At this stage no-one had realised he was impaled, the man said.
Emergency services arrived within 15 minutes, he said, and took over.
"They managed to get him to come round.
"I don't think anyone realised at first he was impaled.
"It might have been 30 minutes since the crash before anyone realised he was impaled", he said, believing it was because the driver's side was tight against the fence and nothing was visible from the other side.
Once the paramedics and firefighters realised a piece of timber had pierced through the car door and into the man's side "everyone sprang into action", he said.
"They tore the car apart, they had a saw, and the jaws of life."When they took him away in the ambulance he was critical."
The car appeared to have been coming up the hill when it crashed, he said.
"He's come in completely sideways, hit the fence and the tree, and kept going into the fence."
The driver was not wearing any trousers at the time, he said.
It was believed he lived nearby.
"Hopefully the guy's alright," the witness said.
"They were quick to rush him off once they got him out."
The man was still in a critical condition in Auckland City Hospital shortly before noon, a district health board spokesman said.
A neighbour living near the property where the crash happened understood the man was impaled in the abdomen, by a piece of fence that had gone through the car.
The car came up Fairview Ave and went through a fence, a tree, and onto a retaining wall, narrowly missing a house, she said.
She understood from another neighbour that the badly injured man was a local who lived with flatmates in the neighbourhood.
"His flatmates said they saw him at about midnight."
The man was then believed to have gone out for a drive, come back and lost control.
Fairview Ave had gotten busier in recent years, she said, but it was too soon to speculate on the cause of the crash.
"We've had a few similar incidents. Somebody drove into a ditch ... I really don't understand."
The owner of the house where the crash happened was out of town, and tenants were in the house, she said.
The Serious Crash Unit was investigating.
