Writing systems back in the old days, it used to be that occasionally one would need to validate suburb/postcode data, or provide users with a choice of suburbs in a particular postcode, and you used to have to go talk to Australia Post to get hold of that data. It was an administrative hassle, and so updates tended to be infrequent, because really, who has the time?
In these enlightened times, they have it freely available on the web. No registration, no delays, just download. Aussie Post, you rock!
For those dabbling in postcodes worldwide, the Universal Postal Union has an index of post office sites, as well as information on the various postcode formats in use. Jakob Nielsen did a column recently on dealing with international names, addresses and measurements.