Ole Wallis was at a nearby gift shop with his partner when he heard, then saw, the drama unfold shortly before 3pm.
''I just heard a loud bang and then we looked out the window and saw the truck coming down in sort of slow motion. We ran there just in case someone was hurt or something.''
Mr Wallis then called emergency services, which had already received several calls about the accident, he said.
Constable Neil Rushton, of Oamaru, said the Timaru-based H & J Bruce Transport Ltd truck was travelling south on Humber St at a speed of about 50kmh.
The driver, who had not driven on that route before, thought his truck would be able to fit under the 3.5m clearance point, Const Rushton said.
While the cab of the truck did squeeze under, its canopy did not and hit the concrete bridge.
The vehicle came to a halt and toppled on to its side and blocked the northbound lane.
''It's going to give you a good shunt over at 50kmh when you're hitting that concrete,'' Const Rushton said.
The driver was shaken but uninjured.
The truck was not loaded at the time.
Firefighters at the scene covered an area under the truck with sawdust to prevent oil spreading on the road.
Kiwirail South Island network services manager Jeanine Benson said Kiwirail staff had inspected the bridge and no damage was found.
The line was quickly reopened and no services were affected, she said.
Humber St was reopened about 5.40pm after the truck was loaded on to the back of another truck and taken back to Timaru.
A similar incident took place in May 2014 when the driver of a truck transporting several cars on a large trailer misjudged the clearance and slammed into the bridge
Three cars, including late-model Audi and a 2014 Ford Kuga, were damaged.
