Postal Areas are ABS approximations of Australia Post postcodes, created by allocating whole Statistical Areas Level 1 (SA1s) on a 'best fit' basis to postcodes.
Census Postal Areas exclude non-mappable Australia Post postcodes such as:
post office box postcodes; some delivery route postcodes, which are also covered by other postcodes (a situation which often occurs in rural areas); and
some postcodes which, because of the application of the 'best fit' principle, do not get an SA1 allocated to them.
This means that there are more Australia Post postcodes than Census Postal Areas.
Every SA1 is allocated one valid Australia Post postcode as the Postal Area for that SA1. When a person is enumerated in that SA1, the Postal Area is allocated to the person as their Postal Area of enumeration.
When a person's address is coded to their SA1 of Usual Residence, the Postal Area of the SA1 is allocated to the person as their Postal Area of Usual Residence.