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Although extensive efforts are made to contact all occupied dwellings and count all unoccupied private dwellings in the Census, locating and contacting them all is not possible. Some dwellings may not be identified. For example, flats above or behind shops or attached to private dwellings may not be included in the Census. Analysis of the undercount in previous censuses has shown that people away from their usual residence on Census Night (for example, travelling, camping, staying in a non-private dwelling, or visiting friends) are more likely to be missed than people at home on Census Night.

Even when a household is contacted, undercount is possible if not all members of the household can be included on the form (six people can be recorded on the paper form and 25 on the online form), and no extra online or paper forms are obtained. Undercount is also possible if the household, or a member of the household, fails to complete a Census form.

A measure of the extent of underenumeration is obtained from the Post Census Review (PCR) (also known as the Post Enumeration Survey (PES)). The official population estimates produced by the ABS take into account the results of the PCR. However, the Census counts are not adjusted.

See also 2021 Census overcount and undercount, Estimated Resident Population (ERP) and Post Census Review (PCR).

Unemployed persons are defined as all those of working age who:

• were not in employment
• carried out activities to seek employment during a specified recent period
• were currently available to take up employment given a job opportunity.

The unit record file (URF) is a sequence of records held on computer files. It holds coded data for all the person, family and dwelling characteristics in each Statistical area level 1 (SA1) as collected in the Census. It is the original source of all Census products. It excludes records for persons listed as temporarily absent, as their details will have been recorded at their place of enumeration on Census Night (if they were not overseas).

Census data are stored in a hierarchy of records for each dwelling. Each dwelling may contain a number of family records. Each of these, in turn, may contain a number of person records. When using household or family data it is necessary to recognise these three levels and understand the concepts at each level.

The three levels are indicated by the last character in the mnemonic for each variable. Dwelling level variables are indicated by D, family level by F, and person level by P.

The URF is held under strict security and is only accessible by certain ABS officers.

See also Data processing and Mnemonics.

A person who lives in a family household, but who is not related to any person in any of the families in the household.

See also Relationship in household (RLHP).

Usual residence data provides information on the usually resident population of an area, and on the internal migration patterns at the state and regional levels. The 2021 Census has three questions on usual residence that ask where the person usually lives on Census Night, and where the person usually lived one year ago and five years ago. Usual address information is used to code usual residence. Population measures based on place of usual residence are also referred to as the de jure population.

See also Comparing Place of enumeration with Place of Usual Residence, Place of usual residence (PURP), Usual address indicator Census night (UAICP), Temporarily absent and Visitors to a household.