It shouldn't have anything to do with population. Australian cars are sold in Australia, New Zealand, the UK, America, UAE, and probably others that I'm unaware of. The Vauxhall VXR8 (HSV Clubsport) is sold in the UK, which has a population of over 60 million... If GT5 really was the car encyclopedia PD claim it to be, they would put the effort into including at least a couple of cars from all the significant markets around the world.
Not only that, but Australia have one of the top top touring car championships in the world, host a round of the Formula 1 and, as mentioned, have one of the best race tracks in the world.
Fair enough if they couldn't get the licences to companies like Porsche, but given the amount of time and money that went into GT5, there's no reason why they couldn't include at least a couple of Australian cars.