Tauranga police acting Senior Sergeant Iain Chapman said a large metal disc on the road damaged the truck.
"This piece of metal has been flicked up by another car and under the truck, causing damage to the truck oil sump," Mr Chapman said. The oil spill was believed to be about 500m long and sand was used to clean it up.
Meanwhile, the 48-year-old truck driver involved in the Kaimais crash suffered serious injuries and was taken to Tauranga Hospital.
The crash closed SH29 near Ruahihi Rd while emergency services worked to remove the truck and clear the logs spread on the road.
The truck had been travelling towards Tauranga when police say it crossed the centre line and crashed into the ditch about 12.45pm.
The road remained closed until about 3.30pm and was down to one lane until 6pm.
The truck's position against the bank hindered firefighters' efforts to free the driver.
Greerton fire brigade Station Officer Dave Whistler said a lot of the time it took to free the man came from trying to gain access to him inside the truck's cab.
Because of the truck's position up against the bank, firefighters and ambulance staff initially reached the driver through the truck's passenger door.
The man's legs were pinned under the truck's steering wheel.
Traffic was backed up beyond Tauriko but was eventually diverted through Omanawa Rd and McLaren Falls Rd.