A quarter century of subnational working-age population change in New Zealand: Contributions of migration and cohort turnover 1998-2003

This paper quantifies the demographic drivers of change in New Zealand's working-age population (ages 15-6)) across 66 territorial authorities and 21 Auckland local boards over 1998-2023. Using Stats NZ population estimates and subnational mortality data, we implement a demographic accounting decomposition in five-year intervals that separates working-age population change into cohort turnover (entries aged 15-19 minus exits aged 60-64), working-age deaths, and residual net migration. Nationally, the working-age population expanded in every period, but the dominant component shifted. Positive cohort turnover accounted for most growth through 2013, whereas residual net migration contributed over 90% of growth after 2013. Subnationally, negative cohort turnover spread from being experienced by no areas in 1998-2003, to a substantial minority of areas by 2018-2023. The number of areas with declining working-age populations fluctuated substantially from one period to the next. A four-category typology and analysis of residual migration offset ratios for areas with negative cohort turnover shows that positive migration offsets negative cohort turnover in some places but not consistently, leaving local labour-supply trajectories increasingly contingent on volatile and spatially uneven migration.