General Motors have some pretty good facilities down here and have been swapping technology between the two countries for a very long time. Mostly for cost cutting.
Also, 5000 new car sales is actually a lot of cars for one model. Especially for big gas guzzlers like the Commodore/Chev SS. No way they sell any where near that amount of Commodores here in Aus, we are probably in the 1000-2000 range. It would probably be cheaper for General Motors to make essentially one car and sell it in both countries, than try and build and sell two completely different vehicles.