OK so I'm not as familiar with aussie as I am NZ but they are both very similar. When I read stories on here or have met foreigners, a very common theme of drug use/abuse, was taking shit loads of quantity of X. For example, when I was doing ecstacy and knew these english dudes, they'd 'brag' I guess you'd say about how many pills they took back in the UK in one night and it was way cheaper. They'd also admit yeah, they were also way weaker but overall it was cheaper to get high. In my days I never took a pill that on its own didn't provide significant effects just as an example. I knew people pretty deep into heroin and it goes for 500$USD a gram here, they didnt do more than a few grams a week because thats all it takes to be full on doped up ya know - it was pretty strong, I smoked it a few times.
I guess what I'm saying is, the drugs cost more but they are higher quality and people do less of them in general. You won't find many people in either of these countries doing cocaine daily, going through multiple grams. But you'll find plenty(well, not really, but not nothing) who can afford and do indulge in multiple grams over the weekend, every single weekend. Because hey, they earn decent money and can afford to blow 1000$ on 'weekend fun'.
A lot of people earn pretty good money in NZ and Aus and have significant disposable income, or have made large chunks of cash off property booms or inheritances and so on.