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REISSUE - WA Postcodes 6798 and 6799 were incorrectly listed as Unallocated Postcodes, these two postcodes have been removed from the list. No other changes have been applied to this product.


Postcodes and Census Data

1 Postcodes and postal areas

A postcode is a four digit number used by Australia Post to assist with mail delivery. As it is readily available on the end of each postal address, many researchers and businesses use postcode as a make shift geographic region for aggregation and analysis of address based data. The desire to compare such data to census data in turn creates demand for census data aggregated by postcode. Census data are collected on Collection Districts (CDs) which, for several reasons, are not compatible with postcodes. The Australian Bureau of Statistics (ABS) has, however developed a Census Geographic Area classification known as Postal Areas which approximate the area of those postcodes where people live.


Postal Area

Postal Areas are created by allocating whole CDs to Australia Post postcode areas on a 'best fit' basis (See Statistical Geography: Volume 2 - Census Geographic Areas (Cat. no. 2905.0) available July 2002). Postal Areas derived in this way only approximate Australia Post postcodes.

In developing Postal Areas each CD is allocated an Australia Post postcode. Allocations are done on the basis of population and not area. This means that a CD is allocated to a postcode if the majority of the population in that CD is contained within that postcode. Postal Areas are then formed by joining together the CDs that have been allocated the same postcode.

Allocations have been determined using the best available information on postcode boundaries. Unfortunately official maps of postcode boundaries have not been updated since the early 1990s and none of the more recent interpretations have been endorsed by Australia Post. Users should, therefore, be aware of the limitations of the Postal Area classification.

Some Australia Post postcodes are not included in the Postal Area classification. For instance, in some cases when applying the 'best fit' principle there is no CD which can be allocated to a particular Australia Post postcode. This occurs in two cases:

• where a CD covers two or more whole postcodes, the CD can be allocated to only one Postal Area ; or
• where more than one CD partially covers a postcode but all the CDs are allocated to other postcodes with which they also share area.

NSW	-	Postal Area 2620 crosses into the ACT, 2406 crosses into Qld and 2540 crosses into Jervis Bay (Other Territories).
Vic	-	Postal Areas 3500, 3585 and 3644 cross into NSW.
Qld	-	Postal Area 4825 crosses into NT.
SA	-	None
WA	-	None
Tas.	-	None
NT	-	Postal Area 872 crosses into SA.
ACT	-	Postal Area 2618 crosses into NSW.
OT	-	N/A

• many respondents mis-report their postcode;
• it is also difficult to say what the 'correct' postcode for some respondents should be, especially when considering post office boxes and rural delivery services. Census data are based either upon the Postal Area of the residence where a person was enumerated or the Postal Area where they normally reside. By contrast many non-ABS datasets only record the postcode of a person's mailing address. Since many people have different mailing and residential addresses the datasets are difficult to reconcile; and
• postcode boundaries change over time and it can take a long time for respondents and coding systems to adjust to a change in a postcode. This causes problems when working with postcode based data taken from different reference periods as the changes to the boundaries are difficult to reconcile with changes in the population.