Australian postcodes were introduced in 1967 by the Postmaster-General's Department (PMG) to speed up mail sorting. Before postcodes, mail was sorted by suburb names only, which could be ambiguous. Many Australian suburbs share names, and suburbs changed names over time. The new numerical system provided unambiguous sorting.
The initial postcode system divided Australia into zones based on state and region. Over the decades, postcodes evolved as new suburbs were created and urban areas expanded. Today, Australia has approximately 2,500 postcodes covering 17,410 populated localities.
Australian postcodes follow a four-digit system. The first digit indicates the state:
The remaining three digits narrow down the location further. Within NSW (2xxx), postcodes are generally organised geographically. For example, Sydney inner suburbs have 20xx postcodes, while outer suburbs have higher numbers.
Australian postcodes have some quirks worth understanding. The Australian Capital Territory has postcodes starting with both 02 and 26. Tasmania starts with 7. Mail delivery postcodes differ from PO Box postcodes in some areas. Some suburbs have multiple postcodes, and some postcodes serve multiple suburbs.
These terms are often confused. A "locality" is the formal administrative unit used by the ABS (Australian Bureau of Statistics). A "suburb" is an informal name for a residential area. A "postcode" is an Australian Post mail delivery zone.
Many suburbs correspond to localities with the same name. But some suburbs span multiple postcodes, and some postcodes cover multiple suburbs. This is why mapping between the three isn't always straightforward.
Inner Sydney suburb Double Bay has postcode 2028. But nearby suburbs Rose Bay and Bellevue Hill also have postcode 2028. All three suburbs share the same postcode. Meanwhile, Sydney CBD (2000) covers multiple suburbs including the CBD proper, areas near Central Station, and parts of Barangaroo.
SEIFA (Socio-Economic Indexes for Areas) is an ABS product that ranks Australian areas by socio-economic disadvantage. The SEIFA Index of Relative Socio-Economic Disadvantage (IRSD) combines Census variables related to income, employment, education, and occupation.
RefDat postcodes.refdat.com includes SEIFA scores for every suburb. These scores help users understand the social and economic context of each location. A score of 70+ indicates lower disadvantage (more affluent). A score below 50 indicates higher disadvantage (lower income, higher unemployment).
The ABS publishes detailed Census data at the locality level. RefDat uses this data to provide demographic profiles for every Australian suburb. This includes age distribution, country of birth, employment status, education levels, language diversity, and family composition.
These demographics are derived from the 2021 Census, the most recent comprehensive survey. Census data is updated every 5 years, so the next update will be in 2026.
When Australian Post defined postcodes in 1967, they didn't align perfectly with suburb boundaries. In dense inner-city areas, multiple suburbs share a single postcode because they're served by one mail delivery centre. In outer suburbs and regional areas, postcodes may be defined by geography rather than suburb names.
Changing postcodes after 50+ years would be disruptive and expensive, so the system has remained relatively unchanged despite urban growth.
Some organisations have a PO Box postcode that differs from the delivery postcode for their street address. This is because PO Boxes are sorted at specific mail facilities. When looking up postcodes, it's important to know which type you need.
Understanding postcodes and demographics is useful for many purposes. You might want to compare suburbs using RefDat postcodes.refdat.com, which provides detailed demographic comparisons. Or check weather.refdat.com to see how climate varies across postcodes, which relates to cost of living and lifestyle preferences.
Australian postcodes are more than just mail codes. They reflect 50+ years of postal history, urban geography, and Census administration. Understanding how they work helps you interpret demographic data, navigate addresses, and understand the geography of Australia.