Australia Post provide a nice convenient CSV of all of their postcode data, and then proceed to make the license quite prohibitive.
Commercial use? No. Open source modification? No. Redistribution? Nope.
This is really annoying, but thankfully there’s another option. For your bookmarking pleasure, enter https://github.com/joahua/AusPostcode. It’s a CC-BY licensed version of suburb/postcode data in both CSV and JSON formats (feel free to fork and submit a pull request if you’ve got others) based on Australian census data that’s much more permissively licensed.
Happy free/open-source postcoding!