A man has been charged following a horrific multiple-injury crash on Auckland's Southern Motorway on Saturday.

The crash took place at 11.30am on the Southern Motorway, between East Tamaki Rd and Te Irirangi Dr, after a vehicle smashed into a parked car at full speed.

The driver of the stationary car had her pelvis broken in two places.

Another person, who was trapped in the car, had her back broken in two places and also broke her leg.

A Police spokeswoman told the Herald that a 50-year-old man had been charged with refusing to provide a blood sample following the crash, while further charges were also pending.

The driver of the station wagon stands next to his car in the aftermath of Saturday's horror crash on Auckland's Southern Motorway. Photo / Sam Sword

He has been summonsed to appear in Manukau District Court next week.

The charges come after a witness described seeing a man driving erratically and looking "totally out of it" before the incident.

The witness, Allan Nicholls, 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," Nicholls 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."

Nicholls said he noted the vehicle's registration 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.

The driver of the station wagon waits with emergency services in the aftermath of Saturday's horror crash on Auckland's Southern Motorway. Photo / Sam Sword

"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."

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

The crash brought traffic to a sudden halt in southbound lanes and caused widespread traffic chaos.

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