Population projections prepared by governments all vary to some degree. Although most governments use national projections prepared by the ABS as a starting point, they then apply their own assumptions within their jurisdictions. In the Audit, ABS projections were used as a starting point for all regions to ensure a nationally consistent approach to the modelling.
The economic analysis used population data from 2011 (which was the most recent national census year) and projections to 2031 prepared by the ABS. The year 2031 was chosen to align with the national census cycle and five yearly population projections commonly used by governments, and to provide data to underpin development of the 15 year Australian Infrastructure Plan.
The baseline scenario was built on ABS medium level projections. In the base case, Australia's population grows by 36.5 per cent from 22.3 million in 2011 to 30.5 million in 2031. The Bureau's high population growth projection was used to model a high population growth scenario. On that projection, Australia's population would grow to 31.9 million in 2031.
Within the national projections, the Audit also used ABS projections for each state and territory, and the capital cities (the Greater Capital City Statistical Area) as 'control totals'. The difference in population between ABS projections for each state/territory and its respective capital city was then distributed between the other Audit regions in each jurisdiction. This method took account of the base year population in each region, and also of the economic structure of the region (e.g. whether particular industries are likely to be growing or declining, either in absolute terms or in that particular region).