Dashcam footage: Steel debris blocks Auckland's Southern Motorway after bridge collision

Two southbound lanes on Auckland's Southern Motorway have reopened after a steel frame on a truck hit the Penrose bridge, leaving debris strewn across the road.

Auckland commuter Jeff He was horrified when a truck driver apparently ignored a flashing warning light on the Southern Motorway this morning, ramming its over-height steel load into the Penrose bridge.

"I couldn't believe it, the driver should have stopped," said Mr He, who was four cars behind the truck as two large steel trusses were dragged off it and strewn across two southbound lanes of the motorway.

Steel debris litters Auckland's Southern Motorway. Photo / Liz Buckley

"The steel frame on the truck is taller than the bridge and came off.

"Before the bridge there was a warning strobe light, it was flashing before the truck hit it."

Mr He, who was on his way to work, said the car front of him appeared to have been damaged by one of the trusses but he did not believe anyone was injured.

Inspector Chris Tate of the police northern communications centre said two motorway lanes had been shut which the trusses could be retrieved, leaving just one open from the crash, which happened at 8.08am.

However, both lanes had now been reopened.