The witness said it was a tragedy for the truck driver and his family.
"There was a really heavy and icy rain starting to fall just as rescue workers turned up. And it looked like he'd slid on the wet road as he came around the corner and crossed the line and went over the bank.
"They and their family would never have thought that this is how Saturday would pan out."
The truck was heading east toward Napier and on the Napier side of State Highway 5 when the crash happened.
He described the truck as severely damaged and said the 100kmh road was down to one lane when the witness drove through.
Police, firefighters, St John Ambulance and a rescue helicopter from Napier attended the scene.
A police spokesperson said it was a single-vehicle crash involving a large truck that went about 10 metres down the bank.
She said the road was not blocked by the crash, but motorists were asked to travel through the scene with caution.