Yes, you do. It’s compulsory to complete the Census. The Census and Statistics Act 1905 says so. Every parliament for more than 100 years has endorsed the idea of a mandatory census. So are Household Surveys compulsory, by the way.
The Australian Bureau of Statistics (ABS), which collects Australia’s formal records, such as the 2011 Census of Population and Housing (which is its formal name), has an excellent track record for maintaining privacy and security of data. The Census information is private and confidential: no-one outside of the ABS (including other government agencies) is permitted to see your Census responses. Also, individuals or households cannot be identified in Census results or data.