A dashcam recording has captured the appalling moment a car speeds up a bus lane and smashes full speed into multiple other vehicles.

Ben Comemadang was driving on Auckland's Southwestern Motorway on Monday afternoon when the bizarre crash unfolded in front of him.

"It was heavy traffic and I saw this car was driving on the bus lane very fast," he said.

"I thought they were running from police, but there were no police after him whatsoever.

"He then came right up to behind the bus. I thought he was going to hit the bus but then he swooped into lane one and hit the Mazda SUV."

The damage done to the Mazda SUV after it was ploughed into by a speeding driver. Photo / Supplied

The footage shows the Mazda spinning around in front of the bus, and ending up in the bush on the left hand side of the road.

"I was shocked. I was like 'woo what is going on?'," Comemadang said.

"It didn't look like he tried to put the brake on or anything. It was weird.

"This car then smashed into other cars in front of the Mazda, and then ended up in the bush as well."

Comemadang said the bus and other motorists pulled over to check the occupants of the vehicles were okay.

"All the bus passengers came out and looked at the victim's car. I went down and made sure the driver and passenger were okay," he said.

"The passenger was out of the car and was very shocked. The driver was still in the car in shock and crying.

"I said you better get out of the car as I can smell gas."

Comemadang said he did not get a chance to see what happened to the driver of the speeding vehicle, but police were contacted.

The Herald has sought comment from the police.