Estimating the size and composition of the Indigenous population begins with the Census of Population and Housing. The Census is conducted every five years, in years ending in 1 and 6. Coverage is generally excellent, but some groups are harder to count than others, such as homeless people and young men. The Australian Bureau of Statistics has a special Indigenous Enumeration Strategy to improve the count of Aboriginal people and Torres Strait Islanders, but the under-count is higher for the Indigenous population than for the total population. For more information, see http://www.abs.gov.au/websitedbs/
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Until it was changed as the result of a referendum in 1967, the Australian Constitution specified that Aboriginal people were to be excluded from official population figures. (Torres Strait Islanders were treated inconsistently from one Census to the next. Sometimes they were categorized as Aboriginal and therefore excluded from census counts; other times they were categorized as European or Polynesian and included in the count.)

Because the census question about Indigenous status served a different purpose before and after the Constitutional referendum, the wording of the question changed. Before 1967, a narrow, "blood"-based definition (using terms such as "full-blood" and "half-caste") was used to exclude people from the count. After 1967, a wider, descent-based definition was used to define a population sub-group.