Sergeant Mmako Mophiring said the men allegedly ordered the drivers of the truck to unload the cars before fleeing the scene with the six cars.
The incident happened on the N5 highway.
The truck was travelling from Durban to Namibia when it was forced off the road by a Nissan Hardbody, Nissan Navara and Mercedes Benz, one of which had a blue light on.
No person was shot or injured during the hijack.
It is not known whether the hijackers fled the scene as a convoy or in different directions.