A man who witnessed a horrific multiple-injury crash on Auckland's Southern Motorway said he was nearly hit by a vehicle before seeing it smash into a parked car at full speed.

The witness was travelling south on the motorway with his wife on Saturday and said they were almost side-swiped by a station wagon just before the Otahuhu off-ramp.

"I gave him a prolonged toot on the horn because he was driving dangerously," the man said.

"He then proceeded to weave in and out of cars ahead of us but later slowed down enough for us to overtake him just after the Papatoetoe turn-off."

The man said he took the vehicle's registration down with the intention of reporting the driver for extremely dangerous driving.

"As we passed, my wife remarked that he looked totally out of it and I could see that he was smoking a cigarette and looking straight ahead, but it just didn't look at all right," he said.

They tried calling the police, and it was then that he witnessed the accident.

"I was observing him in my rear-vision mirror, then to my horror, I saw him drive full-speed into another car that had been parked on the emergency pull-over side of the motorway," Nicholls said.

"I clearly remember saying to my wife, 'that bastard is going to kill someone if he isn't stopped', then I said, 'he's just done it'."

The man said he saw the station wagon slam at full-speed into a car that was parked on the emergency strip on the motorway.

"I saw him hit that other car going sideways from the impact and what was left of his car fly into the air and land in the middle lane of the motorway," he said.

"We both felt quite sick, but with the volume of traffic and how far we'd since travelled since the impact there was little point in trying to go back to see if we could help."

The crash brought traffic to a sudden halt in southbound lanes between East Tamaki Road and Te Irirangi Drive. It caused widespread traffic chaos.

Another witness said a car was flipped over and was really badly crushed on both ends.

Police yesterday confirmed one person had been critically injured in the 11.30am crash.

Three ambulances, two fire engines and the Serious Crash Unit attended the incident.

A police spokeswoman said the police were speaking to a number of witnesses in relation to the crash.

She said the driver of the stationary vehicle remains in hospital in a stable but serious condition.

Another person was trapped in the vehicle and was removed by emergency services, but sustained only minor injuries.

Two other people in the car were "shaken but uninjured".