I see these are built on the Australian Bureau of Statistics "approximate" postcodes which are matched to census districts and therefore more useful for demographic analysis; no CDs cross postcodes in this dataset. There are a bunch of similar geographic datasets available from the ABS, including electoral boundaries, local government areas and so on: http://www.abs.gov.au/websitedbs/D3110124.NSF/24e5997b9bf2ef...

All of that data is approximated by the ABS from whatever body controls the "real" versions, but is 99% good enough for anything you'd want to do with it.

Australia Post, on the other hand, don't really use postcodes to deliver mail anyway, I've been told. They're a vestigal construct which are very prominent in people's minds but so fraught in the implementation that any systems which analyse them (including Australia Post's) work against them rather than with them. I didn't know about Canada's system — that sounds like a much better idea!