Data Collection
Subnational Population Estimates (2018-base)
Methodology
Subnational population estimates
The estimated resident population for subnational areas by age and sex at 30 June are produced annually.
First release of provisional subnational population estimates at 30 June of a reference year generally become available in October of the same year. These estimates will remain provisional for 2 years.
The release of subnational population estimates for a reference year includes:
- provisional estimates at 30 June of the reference year,
- revised provisional estimates at 30 June of the previous year, and
- final estimates at 30 June of the second previous year.
Base population The estimated resident population at 30 June 2018 forms the base population for deriving current post-censal population estimates. The estimated resident population of each area at 30 June 2018 is based on the census usually resident population count from the 2018 Census (held 6 March 2018), with adjustments for:
- people missed or counted more than once by the census (net census undercount),
- residents temporarily overseas on census night,
- births, deaths, and net migration between census night and 30 June 2018,
- reconciliation with demographic estimates at ages 0–14 years.
2018-base territorial authority and Auckland local board (TALB) estimates
TALB estimates are derived using a component method. They are based on the latest Census data and updated to account for births, deaths, and net migration since the Census. The formula used for estimating the resident population in each TALB is: Population (current year) = population (previous year) + births - deaths + net internal migration + net international migration. All components of the formula (on the right-hand side of the equation) are estimates of the number of events that occurred during the current year ended 30 June. The estimates are constrained to agree with the corresponding national estimated resident population at the time of publication.
Births/Deaths
Birth and death registrations are used to estimate the number of births and deaths that occurred during each June year. There is often a delay between when a birth or death occurs and when it is registered, and this delay is accounted for when estimating birth and death occurrences.
Net migration
For subnational areas, net migration includes net gains from international migration (flows into and out of New Zealand) and net gains from internal migration (flows between areas in New Zealand) during the June year. Annual international and internal migration flows are estimated as two different processes. Multiple administrative data sources are traditionally used to derive estimates of subnational migration because:
- there is no direct measure of internal migration in New Zealand other than the periodic Census of Population and Dwellings,
- the direct measure of international migration (via the outcomes-based migration measure) will use integrated address information from administrative data to inform the geographic distributions of the international migration estimates when estimates are final.
International/external migration
From October 2019, international migration estimates have been derived using the outcomes-based migration measure (see Outcomes versus intentions: Measuring migration based on travel histories). The revised subnational estimates at 30 June 2013–18 also incorporated the outcomes-based migration measure.
International migration data (by age and sex) are used to estimate international migration for all ages. Travellers arriving in or departing from New Zealand are classified as long-term migrants according to the 12/16-month rule:
- A migrant arrival is an overseas resident who arrives in New Zealand and cumulatively spends 12 out of the next 16 months in New Zealand.
- A migrant departure is a New Zealand resident who departs New Zealand and cumulatively spends 12 out of the next 16 months out of New Zealand. We use a statistical model to produce provisional migration estimates. Statistics produced using these provisional estimates have uncertainty for 16 months; after this time, we can finalise the classification of all border crossings (according to the 12/16-month rule).
Once migrant status has been finalised according to the 12/16-month rule, these records can be linked to Stats NZ’s Integrated Data Infrastructure to obtain migrant address information. See International travel and migration – DataInfo+ for more details.
For provisional subnational population estimates, we estimate subnational international migration by imputing TALB from observed records of the reference year, using country of citizenship, Māori ethnic group indicator, age, sex, and direction as predictors. Historical geographic distributions of final international migration estimates may be used to inform the provisional geographic distributions of international migration.
Internal migration
The process for estimating annual internal migration to and from TALBs are derived using multiple data sources in a modelled approach. The sources are:
- linked address notifications data (Integrated Data Infrastructure)
- derived net internal migration data from the series of revised subnational population estimates
- 2018 Census data (usual residence one-year ago), and
- primary health organisation (PHO) enrolment data.
SA2 estimates
The SA2 estimates are derived using administrative data while at the same time constrained to the corresponding TA estimates. SA2 estimates are not produced using a component method due to limitations of estimating the migration flows at the more detailed SA2 level.
SA2 estimates are aggregated to regional council areas.
2018-base SA2 estimates: data and methods
The SA2 population estimates by age and sex are prepared using a combination of administrative data sources: primary health organisation (PHO) enrolment data, electoral enrolment (EEC) data, and building consents.
SA2 estimates are initially a combined estimate using administrative population counts of PHO and EEC data. The combined administrative SA2 estimates are adjusted for coverage at the broad age group level based on previous year’s SA2 estimates.
The initial set of SA2 estimates by age and sex represent an update of the previous year’s estimates based on the current year’s PHO and EEC coverage adjusted population data including an update for natural increase (excess of births over deaths). The initial estimates are adjusted to meet the TA constraint at the age by sex level.
Due to varying coverage limitations by age in the administrative sources, the total SA2 population estimates are also derived using an alternative method. The averaged ratio of building consents to net migration for the SA2s over the previous five-year period are applied to current year’s building consents. The estimated net migration for each SA2 combined with natural increase provides an alternate total estimate. The last step will adjust the SA2 areas with large building activities but not seeing sufficient population increases.
Rounding
All published figures are calculated using data of greater precision. Individual figures may not sum to totals due to rounding.
The rounding of estimates of the five-year age groups, broad age groups, and total all ages is determined by the total population size of the geographic area.
Five-year age groups
- For areas with a population less than 2,000, estimates are rounded to the nearest 5.
- For areas with a population of 2,000 and more, estimates are rounded to the nearest 10.
Broad age groups and total all ages
- For areas with a population less than 10,000, estimates are rounded to the nearest 10.
- For areas with a population between 10,000 and 20,000, estimates are rounded to the nearest 50.
- For areas with a population of 20,000 or more, estimates are rounded to the nearest 100.
The rounding of estimates of natural increase, net migration, and population change is determined by the size of components.
- For components have an absolute value less than 1,000, estimates are rounded to the nearest 10.
- For components have an absolute value of 1,000 or more, estimates are rounded to the nearest 100.
