A piece of steel about the size of "two toilet seats" created "bloody mayhem" on Bulls Bridge late Wednesday night.
About six vehicles were damaged in separate incidents when they hit the object, which was in the middle of one lane, near the southern end of the bridge on SH1, Bulls fire chief Brian Carter said.
Nobody was injured and none of the vehicles crashed.
"It was a big piece of steel. It might have been moved around. The car just hit it, bang, he didn't see it. Then all the air bags in the car deployed with that bang."
The car was a type that shut down automatically if there was a collision.
Mr Carter said the Bulls fire brigade were sent out about 11.30pm to remove the object from the bridge and help damaged cars out of the area.
He said there were three cars in the area with damaged tyres or undercarriage, as well as a car, a van, and a truck that were in town with damage, some changing their tyres on the side of the road.
He said the object was "about the size of two toilet seats", or the size of a car wheel.
The object had been "creating bloody mayhem".