A truck driver has been found guilty of dangerous driving for a crash that brought Auckland to a standstill.

Graham John Kennett, 66, ploughed into the Southern Motorway's Penrose overpass on May 9 last year, causing widespread gridlock on roads around the city.

The Taupo man pleaded not guilty to the charge, saying he had never driven further north than the airport and because he'd been under other bridges that day, he thought his load was fine and checked it with his eye, the Auckland District Court heard at Kennett's judge-alone trial last week.

His lawyer, Simon Stokes, said the temporary warning system at the time of the crash was inadequate.

Police prosecutor Sam McErlean alleged Kennett saw the warning sign but moved into the centre lane where the bridge was highest, instead of moving to the shoulder.

Judge Grant Powell said it was "almost miraculous no one was injured when the digger hit the bridge.

At last week's trial Judge Grant Powell reserved his decision on a verdict until this week.

Kennett was not in court for today's hearing. He will be sentenced at the Auckland District Court on March 9.