The vehicle came to a stop before it went into the water, but remained in a precarious position on a rocky incline at the water's edge yesterday afternoon.
The man did not fall into the water and was in a stable condition, Const Bobbette said.
The accident was not work-related.
A heavy truck arrived at the scene while the man was being tended to and was placed on standby to pull the truck out.
A woman having lunch with a group of others at the tavern, who did not want to be named, told the Otago Daily Times she saw the truck moving, but did not realise anyone had been hit until emergency services arrived.
''We just glanced to see the tail end of it quietly rolling,'' she said.
A fire appliance from Palmerston also attended.
Inquiries are continuing.