State Highway 1 has reopened after a milk tanker truck rolled and slid hundreds of metres down the road north of Auckland this morning.
The truck driver survived with multiple injuries after the crash near the intersection of SH1 and Schollum Access Rd north of Puhoi around 4.45am.
The highway was closed and diversions put in place while crews cleared debris and cleaned up thousands of litres of milk fat that had spread across the road.
It reopened shortly before 6pm today, some 13 hour after the crash. A 30km/h speed restriction will be in place for 12 hours.
New Zealand Transport Agency Auckland highways manager Brett Gliddon said the milk fat had solidified and formed an "extremely slippery" surface across three lanes of the highway.
High-powered water blasters were used to clean the road and make it safe for traffic.
Earlier today, while contractors were still working to clear the road, Mr Gliddon thanked motorists for their patience.
"This is a highly unusual situation involving a very slippery substance and the crash area includes a steep 35km/h corner, so it is vital the spill is extremely well cleaned up to avoid accidents once the road reopens."
A slimy, slippery residue congealing on the road was causing mayhem for recovery efforts and the usually busy highway was still closed this afternoon.
Tens of thousands of litres of milk spilled onto the road after the crash.
"...They're still trying to work out how to get rid of that stuff. It's milk fat ... apparently even the vehicles they're using to pick it up with, they're skidding all over the place," Inspector Cornell Klussein of police northern communications said this afternoon.
A witness at the scene this morning said it was a "miracle" the driver survived the crash.
"It's gone basically 500m down the road, nearly gone over a bank ... he was severely trapped," he said.
"It's going to cause absolute, total chaos."
He understood 35,000 to 40,000 litres of milk products spilled on the road.
The truck and trailer were badly damaged, he said.
"The cab's basically gone.
"It's an absolute miracle he survived," the witness said.
The driver's side was wrecked, he said.The witness said he'd be surprised if the driver was wearing a seatbelt - and suggested not being buckled in may have actually saved him in the freak accident.
However, police have not confirmed if the driver was wearing a seatbelt or not.
The witness thought the driver was tossed around the cab and believed if he'd been in the driver's seat the whole time, he'd have likely been crushed to death.
Under the law, some people delivering goods and people in some goods service vehicles are exempt from wearing seat belts.
It's understood the driver had neck, facial and leg injuries.
The Fire Service said it was alerted to an accident involving a milk tanker in one of the Johnstone's Hill tunnels at 4.47am.
Fire Northern Communications shift manager Dallas Ramsay said firefighters discovered the truck and trailer had crashed about 400m south of Schollum Access Rd in the southbound lane.
"There was one person who was heavily trapped," Ms Ramsay said.
The driver was stuck for some two hours being freed.
The Westpac Rescue Helicopter from Auckland landed on the highway about 100-200m from where the truck had come to a halt.
"He was extensively trapped ... for about two to two-and-a-half hours. So the Fire Service did most of the work," a rescue helicopter spokesman said.
The spokesman said the driver was transported to Auckland City Hospital in a "critical but stable condition", while a St John Ambulance spokesman said the driver was transported in a serious condition with multiple injuries.
An Auckland District Health Board spokeswoman said the man was in a stable condition in hospital just before 11.30am.
The police serious crash unit was at the scene, investigating how the accident happened.
The highway was still closed 11 hours after the truck crash, with diversions in place northbound at Silverdale via SH16 and southbound at Warkworth via Woodcocks Rd.
The New Zealand Transport Agency (NZTA) said it was likely to stay closed until 5pm.Police urged motorists to avoid the area if possible, and use alternative routes.
NZTA initially thought the milk and diesel might be removed from the road by 9.20am, but the wrecked truck and its scattered contents proved far more challenging to clear.
Two large cranes arrived at the scene to remove the destroyed cab, which was still on its side four hours after the accident.