A truckie who didn't know the heavy digger he was hauling was 20cm too tall brought Auckland's main motorway to a standstill yesterday.

About 1.15pm, the arm of the digger struck the Penrose overbridge, causing the digger to be knocked off its trailer on to two of the three southbound lanes of the Southern Motorway.

The driver behind the truck slammed on his brakes after anticipating the accident because he saw the flashing signs go off about 100m before the bridge, warning the truck in the centre lane that it was too high to fit under the overpass.

The man, who didn't want to be named, said the arm of the digger smashed into the bridge, and the machine fell into the right lane in front of his car.


He estimated it had overlapped by just 20cm.

The man ran to the truck, pulling the driver out uninjured, but in shock.

Traffic started to build up instantly as emergency services rushed to the scene.

The New Zealand Transport Agency's highways manager, Brett Gliddon, said the incident response team got there quickly.

They were up against the clock because they knew what impact it would have on the network come rush hour.

But because the digger had fallen under the overbridge, the team had to drag it out before they could move it to the side.

At the same time, a group of structural engineers worked to assess whether the integrity of the bridge had been impaired. However, with only one southbound lane open, traffic quickly piled up.

With vehicles backed up more than 10km from Spaghetti Junction in the city, the transport agency urged motorists to avoid the area by using SH20, or to consider staying at work late. By 3.20pm, traffic south was moving at 15km/h.

Heavy traffic on the Southern motorway as seen from the Mt Hobson footbridge looking south. Photo / Supplied
Heavy traffic on the Southern motorway as seen from the Mt Hobson footbridge looking south. Photo / Supplied

The chief executive of the Auckland Chamber of Commerce, Michael Barnett, said that because of one transport company, the city suffered a "massive loss of productivity" due to people missing meetings, goods not being delivered and international visitors getting stuck in the traffic jam.

"All just because someone didn't know the height restrictions on the motorway. The signs quite clearly state height restrictions for anyone carrying goods likely to be oversized; truck operators should know that. But for them to then fail and pass those costs on to the city, it's just appalling."

Listen: Michael Barnett speaks to Newstalk ZB's Mike Hosking

Mr Barnett, who spoke to the Herald while stuck in the gridlock on his way home to Karaka, said the company that caused the chaos should be held responsible.

Mr Gliddon said the company that owned the truck wasn't liable for any loss of profit or productivity but could be charged for any damage.

The Penrose overbridge is the lowest on Auckland's highways network, with a clearance of 4.57m. The legal height requirement for a bridge is 4.25m - meaning anything that hits it is legally overheight.

However, because it is the lowest, it is the only armour-plated steel reinforced bridge on the network. This means "it does act as a first line of defence and protects other bridges", a spokeswoman for the transport agency said.

In the past 18 months, the bridge has been hit six times.

The agency is also upgrading its Variable Messaging System and overheight detection system to make it even more obvious to drivers if they have an oversized load, Mr Gliddon said.

By 4.40pm, the digger had been moved to the side of the motorway and all lanes were open, with residual congestion.