"All of a sudden he went straight into the tree. The impact of the truck hitting a tree sounded like a cannon going off.
"I ran out my back gate and over the train tracks. They couldn't get him out of the truck.
"A lady said she was a registered nurse. She just held his head up through the window and kept talking to him and telling him to focus on her. I take my hat off to that lady.
"By the time they got him out and into the ambulance they were doing CPR on him.
"I can still see his face," she said. "This is something that will live with me."
Mr Brown said speed was not a factor in the crash and no other vehicles were involved.
The police serious crash unit would investigate, with a focus on whether driver fatigue, distraction, or a medical event had contributed to the crash.
The highway was closed between Ellison St and Awatoto Rd for about three hours, requiring diversions during rush-hour traffic.
The driver's identity was known and police would release his name after his family had been notified.
The death would be referred to the coroner.