2. About the Census

Report on the quality of 2021 Census data: Statistical Independent Assurance Panel to the Australian Statistician

An independent view of the quality of statistical outputs from the 2021 Census of Population and Housing



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The ABS conducts a Census of Population and Housing every five years as required by the Commonwealth Census and Statistics Act 1905. Regularly taking a census provides a comprehensive snapshot of the nation and enables the updating and maintenance of an accurate time series of Australia’s official population estimates.[1]

The Census counts everyone in Australia on Census night and asks about where they were on the night (referred to as their place of enumeration), their place of usual residence and numerous other questions. In 2021, Census night was designated as 10 August. While Census night is the reference night in respect of which people are asked to complete their Census forms, there is no requirement for people to complete their Census form on Census night. The ABS emphasised this in 2021 with a message to the public that they had a ‘window’ of time (before, on and after Census night) to respond to the Census, to increase response (for more detail see Section 2.3).

While the public were encouraged to complete their Census in this ‘window’, the online Census form was open for approximately 10 weeks. Paper forms were accepted for as long as practicable to allow as many people as possible to respond, with the last paper form included in Census processing having been received in December 2021.

The Census collects data on a broad range of topics including Marital status, Family size, Occupation, Language used at home, Country of birth, Australian citizenship, Income, and Ancestry. In 2021, two new questions, on Long-term health conditions and Australian Defence Force service, were included in the Census. In addition, a third response category was added to the Sex question, to allow people to identify as non-binary sex. Also, the Ancestry question was expanded to add tick boxes for Aboriginal and Torres Strait Islander ancestries.


[1] Australian Bureau of Statistics (2019). Planning the 2021 Census. Available at https://www.abs.gov.au/statistics/research/planning-2021-census#more-information

2.1 Uses of the Census

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The purpose of the Census is to measure the number and key characteristics of persons and dwellings in Australia on Census night. This provides a reliable basis to estimate the population for each state and territory and local government area.

The Census also provides the characteristics of the Australian population and its housing for small areas and specific population groups. Census data is used by individuals, organisations, and government to make informed decisions on issues that impact the lives of all Australians.

The ABS identifies four main uses for the Census.[1]

  1. Allocation of government funds and support for elections
    The official population estimates, which are based on Census population counts, are used in the process of allocation of Commonwealth funds to state, territory and local governments, including Goods and service tax (GST) entitlements, and to determine the number of seats each state and territory has in the House of Representatives. The GST entitlements of the states and territories will amount to more than $80 billion in 2022-23.[2] The population estimates are also used as part of the process for determining electoral boundaries for both Australia and most states and territories.
  2. Planning and administration
    The Census provides the characteristics of the population and its housing to support the planning, administration and policy development activities of governments, business and other users. For example, these characteristics have been used to inform the planning of new hospitals and schools.
    While information from other sources provides useful input, only the Census can provide accurate information, nation-wide, and for small geographic areas and population groups.
  3. Use in other ABS statistics
    Census data form the basis of many of the ABS’s most widely used products and services. One is the official population estimates, which are updated each quarter. Population estimates are used in compiling the monthly employment and unemployment statistics and the National Accounts. The Census counts of people and dwellings for each geographic area are also used in the framework for selecting the samples for ABS household surveys.
    In recent years, the ABS has sought to further maximise the use of the Census. Along with other Commonwealth partner agencies, it has linked Census data with a wide range of administrative data to create the Multi-Agency Data Integration Project (MADIP), a secure dataset designed to improve and expand the range of statistical and research insights about Australia. MADIP combines data from the Census with information on health, education, government payments, employment, income and taxation, and employment.
  4. Community uses
    The ABS releases Census results in analytical articles, QuickStats, Community Profiles, TableBuilder and DataPacks.[3] Governments, members of the public, businesses, researchers and community groups use these products to find comprehensive information for small geographic areas and population groups as well as for states, territories and the country as a whole. In addition to direct engagement with ABS products, many people access ABS statistics indirectly, such as through media articles and reporting of aspects of ABS findings.


[1] Australian Bureau of Statistics (2011). How Australia Takes a Census, 2011: Uses of Census Data [Government Paper]. Retrieved from http://www.abs.gov.au/ausstats/abs@.nsf/lookup/2903.0Main%20Features132011

[2] Australian Government (2022). Part 3: General revenue assistance’, Budget 2022-23. Available at https://budget.gov.au/2022-23/content/bp3/download/bp3_13_part_3.pdf

[3] These are standard Census products available from the ABS website.

2.2 How the Census, Post Enumeration Survey, and Estimated Resident Population are related

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One of the most important and well-known uses of the Census is its contribution to Australia’s official population estimates, the Estimated Resident Population. The Estimated Resident Population is compiled through a combination of Census population counts, Post Enumeration Survey estimates, and other administrative sources.[1]

The Post Enumeration Survey[2] is a survey of about 50,000 dwellings, conducted soon after the completion of the Census period. It provides an independent benchmark of the completeness of Census counts. Key changes introduced for the 2021 Post Enumeration Survey were the use of a telephone-first interview approach, and the use of the Address Register (a comprehensive list of all known physical addresses within Australia) for the survey frame.

The Post Enumeration Survey interview process determines whether a person in the sample should have been counted in the Census, and the category in which they should have been counted (such as age, sex, or state of usual residence and whether they are of Aboriginal and/or Torres Strait Islander origin). A matching process determines if they were counted, how many times they were counted, and the characteristics they were counted with. This is done by directly linking Post Enumeration Survey persons and dwellings to their matching Census forms (where they exist). This enables estimates of net overcount or net undercount, which are then applied as part of the five-yearly rebasing of the Estimated Resident Population.

The Estimated Resident Population is a population estimate by geographic area, age and sex and is used to inform many planning decisions by governments and investment decisions by businesses. It is used to plan and forecast needs for housing, schools, hospitals, shopping centres, child and aged care services and transport infrastructure. It is used in the National Agreement on Closing the Gap and more broadly in a range of forecasting and modelling. The Estimated Resident Population is also used to guide the distribution of government funds to states and territories, which flow on to smaller areas such as local governments and is a major factor in the determination of electoral boundaries. The calculation and frequent updating of the Estimated Resident Population is part of the statutory role of the ABS. The Estimated Resident Population forms the basis of the population projections made from time to time by the ABS.

After every census, the Estimated Resident Population is recalibrated, or rebased. Rebasing is done by adding the net overcount or net undercount from the Post Enumeration Survey to the new Census population counts. Further demographic and time adjustments are then made before the Estimated Resident Population data is finalised. Further information on this process is provided at Appendix B.

The Post Enumeration Survey is important as it provides:

  • an independent measure of the completeness of the Census counts;
  • a critical component for rebasing of the Estimated Resident Population to the latest Census; and
  • assistance in identifying improvements for future censuses.


[1] For more information on the Estimated Resident Population, see National, State and Territory Population. This can be accessed at: https://www.abs.gov.au/statistics/people/population/national-state-and-territory-population/latest-release [2] Information regarding the structure and methodology of the Post Enumeration Survey can also be found at: Australian Bureau of Statistics (2022). 2021 Census overcount and undercount. Available at  https://www.abs.gov.au/statistics/people/population/2021-census-overcount-and-undercount/2021

2.3 Key features of the 2021 Census

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As in 2016, the 2021 Census was ‘digital first’. Most households received Census instructions in the mail to complete online, while paper forms were readily accessible via an automated phone service or through online self-help. Some areas where there were, for example, higher numbers of older people or areas where previous Census response had been lower, received a paper form upfront. People living in remote areas and most people experiencing homelessness had help from Census field staff.

The enumeration model for 2021 built on the success of the 2016 approach, which was based on mail out areas (areas which meet criteria for residential address mailability) and drop off areas (areas with poor address information or difficult to access by mail). The proportion of mail out areas increased from 80% in 2016 to 85% in 2021.

There were three phases of enumeration for the 2021 Census - Approach, Reminder and Visit:

  • Approach phase, when letters and, in some areas, paper forms were mailed to addresses (85% of dwellings), with deliveries by early August. The letters requested online participation on Census night but with the option of requesting a paper form. The remaining dwellings (15% of dwellings) were visited by a field officer who hand delivered a paper form. People in these areas still had the option of completing their forms online with online access codes printed on the paper forms. The aim of the approach phase was to ensure that all dwellings were ‘enabled’ by Census night, having received either a letter or a paper form by that date.
  • Reminder phase began after Census night with the start of reminder activities to prompt response. The standard reminder was a letter with online access codes mailed to non-responding dwellings in mail out areas.
  • Visit phase, when field officer visits to non-responding dwellings commenced. Visits to non-responding dwellings in drop off areas commenced earlier than in mail out areas because reminder letters could not be sent to these addresses.

A key feature of the 2021 Census was a change in the messaging about when people should complete their Census. In 2021, Census Instruction Letters were delivered to dwellings across Australia with the banner ‘It’s time to complete your Census – online form open now'. This messaging, reinforced by the Census media campaign, subtly encouraged households to complete their Census as soon as they received their materials and therefore before Census night on 10 August. The letters also emphasised 12 August as a date after which the household may receive contact from the ABS. This messaging was designed to provide a ‘response window’, whereas in previous censuses only Census night was highlighted. Encouraging households to complete their Census early gave them greater flexibility, potentially increasing overall Census response. It was also designed to smooth the response over Census night and thereby assist the ABS in its management of operations. Census results show that the ‘response window’ influenced the timing of response in the 2021 Census, with more than 3 million private dwellings responding before Census night, significantly more than responded early in previous censuses (see Figure 2.3.1). While a significant number still responded on Census night (over 2.5 million), the window was effective in prompting early response and smoothing the submission of Census forms.

Figure 2.3.1 Timing of private dwelling response during 2021 Census field operations

Line chart as described in the paragraph above and the description for this image.
The graph has a line showing the number of private dwellings responding to the Census daily from 26 July 2021 to 10 October 2021. Census night is marked on the graph. The graph shows that more than 3 million dwellings had responded before Census night and over 2.5 million responded on Census night.

Note: Data excludes remote area operations.

Strategies were put in place to maximise the coverage and response of specific population groups, geographic areas and dwelling types. Prioritised population groups included Aboriginal and Torres Strait Islander peoples in urban, regional and remote areas, people from culturally and linguistically diverse backgrounds, people experiencing homelessness, people in aged care, incarcerated people and people with disabilities. As in 2016, various strategies were put in place but engagement with stakeholders was a key strategy across all population groups, with the level of engagement scaled up for 2021. Information hubs were a new aspect of face-to-face assistance in 2021, with Census engagement staff setting up information booths in the community to answer questions, issue paper forms and raise awareness.

A Remote Area Strategy was developed to enhance the enumeration of remote Aboriginal and Torres Strait Islander communities across Australia, covering 51% of the country and including more towns than in 2016. A key change for 2021 was the move to an Aboriginal and Torres Strait Islander led engagement model with a strong emphasis on collaboration with communities and local staff employed where possible. In these areas, the Census was conducted, as in previous years, over an extended period to ensure that Census field staff could cover vast areas and help people complete the Census through face-to-face interviews where required. As in previous censuses recruitment difficulties were experienced in some parts of the country, in particular remote areas, and these were exacerbated by COVID-19 restrictions which limited the movement of field staff within lockdown areas and across state and territory borders. An innovation in the management of remote field operations was the use of a dedicated mobile app, called ‘MyWork’, which greatly improved monitoring of operations and the timely response to issues.

Following the events of Census night in the 2016 Census, when the ABS took down the online form as a precautionary measure after experiencing a series of Distributed Denial of Service attacks, and the recommendations that emerged from several subsequent reviews, the ABS prioritised security and performance, as well as data quality and accessibility in the redevelopment of its ‘digital service’ for the 2021 Census. The digital service included a new public Census website, the online form and a range of self-service options to make it easier for people to complete their Census. In preparation for the 2021 Census, the ABS worked closely with the Australian Cyber Security Centre who provided substantial insight into the development of robust census systems. In addition, the ABS welcomed a review by the Australian National Audit Office, taking all the recommendations from that review into account.

Names and addresses continue to be important data collected on the Census form. As a result of the 2021 Census Privacy Impact Assessment, which was conducted by external experts, names of respondents will be retained for 18 months and form addresses retained for 36 months.

2.4 The 2021 Census and the pandemic

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Running a census is a huge undertaking in normal circumstances but conducting one in a pandemic added complexity. While most of the population received their letters or paper forms and participated as planned, COVID-19 lockdowns progressively disrupted some aspects of Census enumeration, reducing contact with households and non-private dwellings such as hospitals and nursing homes.

In response to the emergence of the pandemic, the ABS had developed a Census COVIDSafe step-down plan and framework to manage field staff and community safety during operations. This plan was put through its paces during the major large-scale Census test in 2020, ultimately providing confidence to the ABS, stakeholders and field staff that the 2021 Census could go ahead. However, with more than half Australia’s population in lockdown at some stage during the Census enumeration period, aspects of field operations were significantly disrupted, particularly in New South Wales and Victoria. The ABS’s management of the Census in the pandemic is explained in more detail in Section 4 ‘COVID-19 and the 2021 Census’.

2.4.1 Occupancy determination

Restricted field operations presented new and unique challenges to determine whether dwellings were occupied due to the combination of a ‘no contact’ approach and ways in which expected occupancy has changed due to COVID-19. However, the ABS had developed a probability model for the 2021 Census to improve the accuracy of determining whether a dwelling was occupied or not on Census night. This was used during Census processing when there was insufficient information from the field to make a determination. The model was also used to validate the final occupancy results for the Census.

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