"I'm a little shook up," Mr Wastell (83), who was driving, said after the crash.
"We were pretty lucky, I guess. I should go out and buy a Lotto ticket, or three."
They were travelling south along Humber St shortly after 11am towards the concrete railway overbridge when he saw a truck coming north carrying a container.
"I didn't think he'd get under the bridge," Mr Wastell said.
And he was right.
The container, bolted to the truck's deck, hit the top of the bridge, causing the truck to tip sideways on to the couple's car.
It hit just above the driver's door, scraping along the top edge of the roof and ripping the rear (hatchback) door out.
Mr Wastell did not see the impact because he was sitting low in the car, but he heard "a horrible crash".
The car's momentum took it out from underneath the container, and he stopped the car about 10m along the road.
Neither he nor his wife was injured.
Mr Wastell said they might not have been so lucky if they had been a split second later passing under the bridge.
Witness Graeme Halliwell, from Timaru, and staff from Placemakers helped get the driver, who was not injured, out of the cab.
The articulated truck, belonging to Central South Island Helicopters, was carrying a container holding water and empty drums.
A small amount of diesel from the truck spilled on to the road and into the Oamaru Creek.
The truck's engine was still running, belching smoke, and it took some time for firefighters, working from the underside of the truck, to turn it off.
The bridge has a sign on the north side marking clearance at 3.5m, although it is partly obscured by graffiti.
There is no warning sign on the south side of the bridge.
Marks on the bridge indicated the container was about 0.5m higher than the clearance level.
The south end of Humber St at Itchen St was closed until about 4.30pm.
