St John Tauranga operations team leader Ross Clarke said it was clear the man had died although ambulance staff were not able to get to him.
"We actually can't get to him but what we have seen doesn't look conducive to life," he said.
Firefighters used pneumatic air bags and hydraulic rams to lift the truck off the man once investigators from the Ministry of Business, Innovation and Employment (formerly Department of Labour) arrived.
The ministry confirmed it was investigating.
Firefighters remained at the farm for almost three hours.
Police Commercial Vehicle Investigation Unit Sergeant Dennis Hogan said his team was investigating the accident alongside the ministry but exactly what happened was not yet clear as there were no witnesses.
"All we know is the truck's rolled down the bank and the driver has been ejected from it."
The truck was removed from the scene yesterday afternoon.
Spreadmaster Transport could not be reached yesterday and residents at the farm refused to comment.