The accident forced diversions on to "very, very user-unfriendly" roads, said Waikato police communications manager Andrew McAlley.
"It's not very pretty, particularly if you've got kids in the car."
Highways north and south of Auckland were crowded as thousands left the city for their New Year holiday.
Auckland Arts Festival Trust chairwoman Victoria Carter was among those caught driving north. A frequent user of the road, she said she had never known the queue to Warkworth to be as long.
"We got to the (Johnstones Hill) tunnel at 11am and there was a queue coming out of the tunnel as we arrived at it and we were hoping it was not the queue for Warkworth ... and it was.
"We crawled to Warkworth at an average speed of 15-20km/h ... It looks like the congestion stemmed from the traffic lights in Warkworth."
Transport Agency spokesman Anthony Frith said a 20km northbound queue formed on State Highway 1 to Warkworth from 10am.
Last month, the Government approved a fast-track consent process for a $760 million extension of the Northern Motorway to Warkworth.
The Transport Agency has not set a start date for the 18.5km four-lane extension from the Johnstones Hill tunnels, but construction is expected to start between 2015 and 2019 and end between 2020 and 2025.
Police urged motorists leaving Wellington to keep calm after delays of more than an hour, and pockets of congestion as far north as Waiouru.
National road policing manager Superintendent Carey Griffiths, who was driving north of Wellington for a barbecue yesterday afternoon, said it was like a "rolling car park".
"It's very, very slow, nose to tail - it took two hours to get from Lower Hutt to Paraparaumu."
He said people had mostly coped with the delays well.
"We still get issues with the odd impatient person and people who just do outright stupid things, but the general feedback is that overall driver behaviour has been good."
By yesterday, there had been one holiday-period road death - Sandy Paul Waenga Brown, 21, from Paua, who died in a crash in the Far North early on Christmas Day.
Police believe speed was a factor in yesterday's Coromandel crash.
Senior Sergeant Freda Grace said the driver appeared to have failed to negotiate a sign-posted 25km/h corner, crossed the centre line and careered down a 20m bank.
"The six occupants of the van suffered a variety of injuries and members of the public had to travel from the scene to raise the alarm because of poor cellphone coverage.
"With thousands more vehicles on the narrow Coromandel roads over the next few days, it's crucial drivers pay attention and slow down."
- Additional reporting: APNZ
