In 2019, Treasury published population projections for the State of Tasmania out to the year 2067, and for local government areas out to 2042. These projections were modelled and "jumped off" from the latest complete population estimates at that time, for 2017.
Treasury generally prepares new projections every five years. However, following the results of the 2021 Census of Population and Housing, the ABS made unusually large adjustments to its Tasmanian population estimates for the period 2017 to 2021 as part of its post-Census rebasing revisions. As a result, the projections Treasury prepared in 2019 became somewhat obsolete.
As a result, Treasury has prepared interim updated population projections by adjusting the growth rates implied by the previous projections and applying these to the latest, rebased ABS population estimates using 2021 as a new jump-off point (Chart 1).
Chart 1 - Total Tasmanian population, and rebased projections, as at 30 June
Source: ABS Cat No 3101.0, and updated Treasury calculations.
In updating these projections, Treasury has calculated the implied inter-year growth rates from the original projection series for each LGA, year, single year of age and sex, and applied these growth rates to the rebased actuals from the 2021 jump-off point onwards.
For the calculation and application of the growth rates, the per cent change was taken from one age (A0) in one year (Y0) to the age of one year earlier (A-1) in the previous year (Y-1). This per cent change was then applied to A-1 in Y-1 of the new projections to get the new A0 at Y0. This measure captures the rate of change from a particular cohort (say, from males aged 25 in 2030 to males aged 26 in 2031) implied by the original projections, and assumes that this rate of change still applies to the new set of numbers.
The treatment of births (persons aged less than one year) needed to be different as there is no A-1 when A0 is births. To deal with this, the births from the original projections were used to represent the rebased births from 2022 onwards.
Treasury will prepare full updated projections for publication in 2023. Further information and expected dates for the new projections will be advised at a future point on Treasury's website.
The updated projections should be used with caution. Please note that the following should be considered in interpreting or using the interim projections:
Population projections basic interim adjustment (the interim projections) - caveats
- The adjustment method has been deliberately kept as simple as feasible. This has advantages in terms of speed and transparency, but the adjusted projections also have significant limitations, particularly in regard to the consistency with assumptions previously made for the 2019 projections. No specific allowances are made for changing fertility, mortality or migration rates that are implied in the revised ABS estimates of Tasmania's population.
- While no specific allowances have been made for changing assumptions, due to the calculation of the growth rates, new births (persons aged 0) were not able to be calculated. To deal with this, the original birth projections were used from 2022 onwards. The rebased projections have the same births as the original projections from 2022 onwards. However, the fertility assumption is not consistent with the updated projections due to the differences in total women per year, and per age, compared with the original model.
- The main objective in undertaking a basic interim adjustment is to address the most immediate limitations of the 2019 projections, which have arisen from the ABS 2021 Census rebasing exercise. This provides users with projections that can be referred to in the short-term while Treasury develops revised, more robust projections based on the new ABS population estimates.
- Users of the interim projections should refer to the documentation relating to the original 2019 Treasury projections for information on the original methodology, assumptions and features of the model and the original projections. However, the interim projection adjustments should be regarded as a separate exercise, noting that the assumptions applying to the original projections will not be internally consistent with the new interim projections.
- The interim adjustment method is not necessarily endorsed by the vendor or developer of the original projection model. The interim projections, and any associated shortcomings, should not be regarded as being outputs of this model.
- While Treasury considers the interim projections to be more suitable for use than the original projections given the changes arising from ABS rebasing, users of the interim projections should consider the above limitations and whether the interim projections are fit for any purpose intended.