Some people in parked cars were injured and the Fire Service had to free one trapped person, Inspector Lou Alofa Alofa said.
Two people were taken to hospital, one with serious injuries and the other with moderate injuries.
Eye witness Paul Scotting said a fully laden truck and trailer had hit a van doing a U-turn on the busy arterial road, then smashed into another truck unloading a Porsche at Continental Car Services.
That Porsche tipped off the truck into another car at the yard, he said.
"It's carnage," he said.
Mr Scotting said the van involved in the initial collision was shunted across the road into the side of a parked BMW.