A single vehicle crash blocked traffic in Wellington's inner-city Terrace Tunnel today.
Video from the scene shows a number of motorists helping push the van to the side of the tunnel.
It appears the vehicle hit the side. It is not clear at this stage what caused the crash.
New Zealand Transport Agency spokeswoman Felicity Connell said the crash had been cleared from the tunnel by 12.10pm.
She said traffic backlog from the crash itself wouldn't take long to clear, however given the Christmas rush due to hit the roads drivers should expect delays.
"It is a busy time on the network at the moment so people need to be patient," she said.
Wellington woman Ash Moor was driving through the tunnel to take her parents to the airport for Christmas when the van flipped just two cars in front of her.
"Mum said, 'Look, a car's hit the side barrier and flipped!’" Ms Moor said.
"Next thing you know the whole tunnel completely filled with smoke.
"It was pretty spooky, it was white smoke, you couldn't see anything at all."
Ms Moor said passengers from nearby cars ran to help the van, including her father who can be seen in the video helping to flip the van the right way up.
Ms Moor said the van's two occupants were "in a lot of shock", but were unhurt.
"[They were] sitting down, quite overwhelmed with what happened."
Despite the major disruption to traffic on Wellington's busiest tunnel, Ms Moor said she was able to get her parents to the airport on time to get their flight for Christmas.
NZTA regional performance manager mark Owens said the crash caused "superficial" damage to the lining of the tunnel wall along the southbound lane.
Mr Owens said the damage wouldn't affect the tunnel's operation and would be replaced early next year.