A man in his 20s remains in a critical condition after a truck plunged off a steep bank, leaving a person trapped on Sunday.

Emergency services attempted to free the driver of the large truck after it crashed about 9.30am off the Napier to Taupō highway at Te Haroto.

A motorist who came across the scene just after the crash said the large commercial truck plunged down a steep 10-metre bank leaving the driver pinned in the cab.

He said the driver had been trapped for about 30 minutes before emergency services arrived, but he had been immediately attended to by an off-duty ambulance officer.

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A Hawke's Bay District Health Board spokeswoman said on Monday a man in his 20s remains in a critical condition.

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.

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She said the road was not blocked by the crash, but motorists were asked to travel through the scene with caution.