A police spokeswoman said the truck carrying a digger struck an overbridge near the Green Island off-ramp about 1pm yesterday.
A Hato Hone St John spokesman said it was notified but not required to attend as no injuries were reported.
A group of V8 enthusiasts from the DV8 Car Club had been enjoying the good weather in Orange St, beside the motorway.
They heard a loud bang f and their car park was showered in dust and gravel.
It sounded "like a skip truck coming down the road," one said.
On the road, broken stanchions could be seen on the ground beside the digger, which had its arm hanging askew.
Lookout Point Senior Station Officer Peter Brosnan said two Fire and Emergency New Zealand appliances attended and assisted with traffic management.
He was pleased and appreciative that people had driven slowly past the crash site, which made life a lot easier for emergency services.
Engineers had been called to check the bridge, SSO Brosnan said.
"His boom was right up," Mrs Harradine said about the digger.
It took out phone lines for three houses. She called Chorus to inform the company of the situation.
Her husband had been watching TV at the time, and remarked that he hoped the digger did not hit something.
Five minutes later, they heard sirens coming from the Southern Motorway.
"You could tell something was going to happen, the way he was going," Mrs Harradine said.
There were subdivisions going in on the hill above them and lines being taken out by trucks had happened before.
The truck was understood to be operated by Southern Transport, while the digger was marked SouthRoads. The two companies are owned by H.W. Richardson Group.
Southern Transport declined to comment about the incident.
