@xssfox phew! Had to trawl the whole thread to see if anyone quite mentioned:
- not just that postcodes change but they quasi-cease to exist, which is to say they get dropped from the standard list you can get from AusPost.
Of course, in reality mail posted to such addresses will still get there, so the list is merely the current "please use these" list.
Anyone with historically collected addresses will have data with postcodes no longer listed.