## Data Collection

## Data Collection

^{en-NZ}

## Methodology

Projection assumptions for fertility, mortality, and migration are formulated after analysing short-term and long-term historical trends, information provided by local planners, and government policy. The Subnational population projections, projection assumptions, 2018(base)–2048 table in NZ.Stat provides a summary of the low, medium, and high projection assumptions for each area.

**Main changes from the previous 2016–base national population projections**

Deriving the projections involved a review of all projection assumptions. The following are the main changes at the national level from the previous 2016–base projections.

- The base population was 4.9 million at 30 June 2018 (2018–base population estimates). This is 89,200 (1.8 percent) higher than the 2016-base median projection for 2020. See Estimated resident population (2018–base): At 30 June 2018 for more information about the base population.
- The long-term median period total fertility rate (TFR) is assumed to be 1.65 births per woman, a decrease from 1.85 in the 2016–base median assumption.
- Age-specific death rates continue to decrease, but this decrease is more gradual compared with the 2016–base median projections. Median life expectancy at birth for males and females is assumed to increase from 80.8 and 84.4 years in 2021 to 84.6 and 87.8 years in 2048.
- The long-term median annual net migration gain is assumed to be 25,000, an increase from 15,000 in the 2016–base median assumption.

**Geographic classification**

The population projections in this release are based on the regional council, territorial authority, and Auckland local board, classifications and boundaries at 1 January 2021.

**Fertility**

The assumed fertility rates are based on estimated births for each area during the period 2017–2020, with change between 2019 and 2048 consistent with the fertility assumptions from the National population projections: 2020(base)–2073. Under the medium fertility assumptions, the period total fertility rate at the national level is assumed to be 1.65 births per woman throughout the projection period. In 2019–2023 the medium fertility assumption ranges from 0.99 births per woman for Queenstown-Lakes district to 2.63 for Opotiki district. In 2044–2048 the assumed total fertility rates range from 0.87 births per woman for Queenstown-Lakes district (low fertility assumption) to 2.79 births per woman for Opotiki district (high fertility assumption).

A sex ratio of 105.5 males per 100 females is assumed, based on the historical annual average at the national level.

**Mortality**

The assumed mortality rates are based on estimated deaths for each area during the period 2017–2020, with change between 2019 and 2048 consistent with the mortality assumptions from the National population projections: 2020(base)–2073.

Under the medium mortality assumption, period life expectancy at birth at the national level is assumed to increase from 80.8 years for males and 84.4 years for females in 2021, to 84.4 years for males and 87.6 years for females in 2048. In 2019–2023 the assumed life expectancy at birth ranges from 74.7 years for males and 78.5 years for females in Wairoa district, to 85.7 years for males and 88.7 years for females in Queenstown-Lakes district. In 2044–2048 the assumed life expectancy at birth ranges from 77.0 years for males and 80.6 years for females in Wairoa district (high mortality assumption), to 89.9 years for males and 92.8 years for females in Queenstown-Lakes district (low mortality assumption).

**Migration**

Migration at the subnational level has both an internal (to/from other areas of New Zealand) and an external (to/from overseas) component, although these separate components are difficult to quantify. The assumed net migration (both external and internal) for each area is based on considering observed net migration during each intercensal period from 1996 to 2018, estimated annual net migration from 2019 and 2020, the capacity of the area for further growth (for areas with net inflow), whether historical outflows can be sustained (for areas with net outflow), the desirability of the area to new migrants, and information available from and about local authorities relating to current and future developments that may affect population change.

Under the medium migration assumption, annual net migration at the national level is assumed to be 10,000 in 2021, 20,000 in 2022, and 25,000 in 2023 and beyond. However, the median assumption is not an assumed trajectory, but the median of the migration simulations that vary around that median.

All TA areas except Westland district are assumed to have different net migration levels during 2019–2023 than the subsequent five-year periods. Different levels are generally assumed for areas susceptible to changes in external migration, such as university cities (which can attract large numbers of overseas students in some years), and areas where constant net outflows are considered unsustainable.

The low and high net migration assumptions are chosen to represent plausible alternative migration scenarios for each area, so are not constrained to any national-level projections.

The age-sex patterns of net migration for each area are based on observed intercensal net migration patterns during 1996–2018, see Subnational population estimates: At 30 June 2020 (since 2018), and the latest migration assumptions from the National population projections: 2020(base)–2073.

^{en-NZ}