The May 9 crash caused chaos for commuters, significantly lengthening journey times and delaying some public transport services by up to 30 minutes.
The Ellerslie-Panmure southbound onramp and two motorway lanes were closed for about three-and-a-half hours after the crash.
At 5.11pm that day traffic on the motorway was crawling at 7km/h between the Fanshawe St offramp and Hobson St onramp.
Today Powell said it was "almost miraculous" no one was hurt when the digger hit the bridge.
For a driver to be held legally culpable for dangerous driving the prosecution had to prove beyond reasonable doubt that the driving could be "objectively" deemed dangerous, the judge said.
He found Kennett guilty because he was satisfied the evidence indicated he had driven dangerously.
Kennett was not in court for today's hearing. He will be sentenced at the Auckland District Court on March 9.
Earlier
At the February 17 trial the court heard how Kennett's flatbed truck passed through a sensor beam and set off a warning sign 288m before the bridge telling him to "pull over" because his load was too tall.
However, Kennett's 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.
Kennett admitted he saw the message but said he didn't realise it was directed at him because other big trucks were travelling nearby.
He accepted it was his job to measure the height of his load before travelling, but he said he didn't do so when he put the digger on his truck in Avondale because he didn't have any equipment.
Instead, Kennett said, he estimated its height by comparing it to the height of his cab, which he knew was 3m.
The height limit for the motorway network at the time of the crash was 4.25m and the Penrose overpass - the lowest on the route - stands at 4.41m.
The height of Kennett's load that day remains unclear, Powell said in his judgment. However, police officers who examined the scene estimated it was more than 4.53m tall.
A man who was driving in the right-hand lane next to Kennett when the smash happened told the court he remembered hearing "an almighty bang".
"I was thinking in my head straight away, do I stay or do I go. I just accelerated through everything," the witness, Hayden Bishop, said.
The court was shown videos of the overpass strike from different angles.
The bright yellow digger could be seen slamming into the overpass in a cloud of dust as the machine tumbles onto the road, narrowly missing Bishop's car.
Defence witness specialist traffic engineer Don McKenzie said he analysed the overpass, traffic data and crash history and told the court he believed the temporary signs were insufficient and "poorly placed".
He said the temporary sign, 288m before the overpass, didn't provide drivers with oversized loads with enough time to realise it referred to them, move out of traffic and stop before the bridge.
In his opinion the hard shoulder was also too narrow for heavy vehicles to stop safely.
McKenzie also analysed information for all 21 recorded strikes of the overpass in the past 10 years as well as the crash involving Kennett.
All were a result of drivers being unaware or unable to react to the warnings, he believed.
However, under cross-examination by McErlean - who called that opinion "a bit sweepy" - he accepted some drivers were aware they were too high but chose to "run the gauntlet".
Today Powell said he had not given McKenzie's evidence much weight when deciding whether to convict Kennett of the dangerous driving charge because he believed the defence had failed to prove the the other 21 recorded strikes were comparable to the case.
