Pontiacs sold here in 1960s tended to have French-sounding model names such as Laurentian and Parisienne so I guess they were sourced from Canada. It's a bit before my time, but I understand that Australia gave a lower tax to vehicles (or CKD kits) imported from Commonwealth countries, so there would have been a tax advantage to GM sourcing them from Canada instead of USA. They were Australianized to a degree - some local content, amber blinkers at the rear (often a cheap looking add-on round light) and of course RHD. I'm not at all up on it, but I have read some experts in these cars saying that Aussie assembled cars were often a mix-up, with last years's dashboard or similar.
Pontiacs and Chevys disappeared from here when Holden started making the Holden Brougham and later the Statesman. They were a niche item any way, sold in tiny numbers in the 60s as they were getting too big for most Aussies' taste.