He tried to avoid it but the unit ran over it - lodging it underneath.
"All he could do was put the brakes on."
The unit skidded across the highway as he tried to scrub off speed and it crashed through roadside barriers and across about 60m of grass and gravel into the front of the Wishart Estate building.
The impact with the building's verandah left the cab crushed and flattened.
"He could have been killed - he was lucky to walk away from this," Mr Miers said.
He had been able to scramble out of the cab and raise the alarm.
Police at the scene agreed the man was lucky to have only received minor injuries, and said the crash could have been worse as a vehicle had been coming the other way at the time.
Mr Miers said the driver was shocked and had been taken to Hawke's Bay Hospital to get the cut treated.
He was discharged about four hours later and was taking time off to recover.
"I'm going buy him a Lotto ticket and take him out for a beer later," Mr Miers said.
While the truck unity was a write-off all six cars being carried were undamaged and a second unit would be brought in to get them on their way again.
The Wishart Estate building escaped damage, with a custodian saying only the verahah section had been torn away.