Central Otago sub area commander Senior Sergeant Ian Kerrisk said a Toyota Land Cruiser towing a trailer and travelling downhill crashed and hit the rock wall beside the highway just before 2.30pm.
The crash appeared to have occurred after the trailer started swaying under its heavy load.
The driver was not injured. Police closed SH8 and redirected traffic until 4pm.
An investigation was under way, but the driver was lucky, as there was potential for a ''much worse outcome'', Snr Sgt Kerrisk said.