Police have launched an internal investigation after footage showing a sickening head-on crash that closed Auckland's Southern Motorway for hours was leaked.

The chilling moment a stolen car hurtled the wrong way along the motorway on Thursday morning, crashing into an innocent motorist, was captured by police on camera.

The stolen car was being followed by the police Eagle helicopter at the time of the crash, which closed all the motorway's southbound lanes.

Four people were injured in the head-on collision.

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Auckland City District Commander Superintendent Karyn Malthus said the footage was "disclosed outside of police without authority".

"We took immediate steps to ascertain how this footage was made public, and an employment investigation has commenced.

"Police take these matters seriously and we are disappointed at what occurred this morning."

The aftermath of Thursday morning's crash on Auckland's Southern Motorway. Photo / VMP
The aftermath of Thursday morning's crash on Auckland's Southern Motorway. Photo / VMP

The vehicle was being monitored at a distance by the Eagle helicopter - and travelling the wrong way towards southbound traffic - when it crashed into another vehicle.

The crash took place about 10am. All motorway lanes reopened about 12.30pm.

Footage of the crash shows the blue stolen car enter the Southern Motorway via an off-ramp - and hurtling the wrong way towards oncoming traffic.

The car managed to swerve past several vehicles but then slams on the brakes just before colliding with an innocent motorist's vehicle.

A truck driver who was getting off at the Highbrook Drive off-ramp told the Herald he got a shock when he saw a blue car heading towards him.

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"The idiot was going down the wrong way of the motorway. He just came flying past on the slow lane - but the wrong way. It was looking like he was trying to find a way to get off.

"I thought: 'Oh here we go'."

The crash blocked all southbound lanes of Auckland's Southern Motorway. Photo / Debbie Bayes
The crash blocked all southbound lanes of Auckland's Southern Motorway. Photo / Debbie Bayes

The truckie said he looked in his mirror just in time to see the crash.

"It just exploded - there was this massive explosion when he hit the other car head-on. It wasn't good to see."

The driver of the blue vehicle had been sitting low in the seat and did not appear to be wearing a seat belt.

He was going so fast the truck driver said he had no time to react - whether it be to beep the horn or call out to him.

The truck driver said he saw all kinds of driving incidents and bad behaviour on the road "all day, every day'' but that even an hour after the crash, he was still thinking about it.

"I'm still shaking. I need a beer.''

Police said they were not pursuing the car at the time of the crash.