Australia Post's postcode data file is the most accurate and up-to-date list of postcodes in Australia. Every known postcode is included, along with state, territory and suburb (locality) information.
You will need a licence to use the Australian postcode list for commercial or business purposes.
The raw data is provided in CSV format, so you can easily integrate it into your systems and tools.
This free PDF enables you to look up postcodes at your convenience, and is for non-commercial use only.