Between the Censuses in 1991 and 1996 New Zealand's de facto (census night) population grew by 246,400. Two thirds of this increase (156,300) can be accounted for by the balance of births over deaths -- natural increase. There was a "recovery" in births in the early 1990s due to a mini baby boom, or "baby blip" (Pool and Bedford, 1996; 1997). The effect of this was to produce the largest annual contribution by natural increase to the New Zealand population since the year ended March 1974 during the year after the 1991 Census (Statistics New Zealand, 1996, 34). Through the first half of the 1990s natural increase remained at levels higher than any recorded since the mid-1970s.

4.1.1  Variations in estimates of net international migration

The balance of the population growth between the two censuses is due to international migration. According to the estimate of inter-censal growth between 1991 and 1996 this balance is 90,300 people -- the difference between the total 246,600 increase and the 156,300 that can be accounted for by natural increase. The arrival/departure data for the five years between March 1991 and March 1996 give a lower figure of 75,800 for this net gain through international migration (Statistics New Zealand, 1996, 34; Lidgard and Bedford, 1997, 38).

The difference between these two estimates of the net migration gain can be explained by a combination of factors associated with the difficulties of linking census data (which are cross-sectional, and are subject to some error through under-counting) and data on births, deaths, arrivals and departures (which are recorded on a continuous basis, and are subject to some errors through under-reporting or, in the case of the arrival/departure data, sampling error). These issues will not be discussed further here; they are documented in publications produced by Statistics New Zealand (see, for example, Statistics New Zealand, 1996 and 1997a).

When examining the respective contributions of natural increase and migration to population growth at a sub-national level it is necessary to take account separately of internal migration and overseas migration. Estimates of net internal migration in particular urban areas or regions can be obtained from the census (see previous section). Estimates of natural increase can be obtained from the Demographic Analysis Section of Statistics New Zealand. An estimate of the contribution made to inter-censal population growth in a city or region by international migration is found in the residual that is left over once the natural increase and internal migration have been accounted for.

4.1.2  Components of growth: four MUA compared

To illustrate the significant variations between urban places in the factors which account for their recent population growth, a comparison is made of the contributions which natural increase, internal migration and international migration have made to the inter-censal increases between 1991 and 1996 in Auckland, Wellington, Christchurch and Tauranga (Table 4.1). Auckland is included for the obvious reason that it is the largest and most cosmopolitan population which is more heavily impacted by international migration than other cities.

Note: Estimates for natural increase for Auckland, Wellington and Christchurch were supplied by Statistics New Zealand (Khawaja, pers comm., 30 October 1997). The figure given for natural increase for Tauranga has been estimated by the author.

Wellington and Christchurch are included because they have very different population structures (Wellington's is much younger than that in Christchurch), which affects natural increase. Both cities received roughly equivalent numbers of internal immigrants (38,900 and 42,500 respectively) and overseas immigrants (21,400 and 20,500 respectively). Tauranga has been selected because it had one of the highest inter-censal rates of population increase between 1991 and 1996, and it is an important destination for internal migrants.

When the respective shares of growth are converted into percentages the differences in significance between the three processes are brought more sharply into focus. Natural increase accounted for 47% of Auckland's population growth during the five years, while international migration accounted for the rest (53%). Auckland actually lost population through internal migration.

In the case of Wellington, natural increase accounted for virtually all the growth while also compensating for the largest net migration loss to other parts of New Zealand of any MUA (Table 3.4). International migration made a surprisingly small contribution to overall growth in Wellington's population, even though the city did have 21,400 people in residence in 1996 who had been living overseas in 1991. It appears that the city also lost around 21,000 people to overseas destinations during the five years.

The data for Christchurch show that the population of this MUA was more heavily impacted by international migration than Auckland during the early 1990s, at least in so far as the share of inter-censal growth that can be accredited to migration from other countries. Almost two thirds (63 percent) of Christchurch's growth was due to net international migration with only one third (33 percent) coming from natural increase.

The much lower level of natural increase (8,000) for Christchurch than is found in Wellington (17,200) is due to the fact that Christchurch's population has a much older age structure than that found in the capital city. Even though Christchurch attracted over 30,000 immigrants from other parts of New Zealand, it lost almost as many people through internal out-migration. Net internal migration contributed the equivalent of only 4 percent to the inter-censal growth in Christchurch's population between 1991 and 1996.

The major contributor to inter-censal population growth in Tauranga was internal migration. Just under half (48 percent) of the total increase in population between 1991 and 1996 can be accounted for by a net migration gain from other parts of New Zealand. International migration contributed a further 27 percent of the growth, while natural increase is estimated to have added the remaining 25 percent.

The net gain from international migration is surprisingly high given that the total number of overseas immigrants to Tauranga was only 4,000, according to the 1996 Census. Almost half (48 percent) of Tauranga's overseas immigrants were New Zealanders returning after a period of residence overseas. This is a much higher share of return migrants in the overseas immigrant flows than was found in Auckland (19 percent), Wellington (32 percent) or Christchurch (33 percent).

4.1.3  Age and ethnic composition

This preliminary assessment of the varying contributions which natural increase, internal migration and international migration have made to recent population growth in four cities serves to illustrate the considerable diversity in sub-national population dynamics in New Zealand. This point has been stressed by Pool and Bedford (1997) in their overview of population change in New Zealand; it is something which commentators at the local level sometimes fail to take into consideration when assessing differential rates of growth in cities or towns of comparable size.

Variations in age and ethnic composition are extremely important when it comes to explaining differential rates of population growth at a sub-national level. Age composition has a profound impact on natural increase, as well as patterns of internal migration. Ethnic composition impacts on natural increase in two ways: firstly through differences in fertility levels between ethnic groups, and secondly through differences in age composition of ethnic sub-populations. Ethnic diversity, along with younger population structures contribute significantly to the higher rates of natural increase in MUA such as Auckland and Wellington, compared with MUA such as Christchurch and Dunedin.

The population projections for Territorial Authority Areas, which have just been released by Statistics New Zealand (1997b), reveal considerable diversity in levels of increase (or decrease) in populations which have a roughly comparable size in 1996. These differences are due in part to variable impacts of internal and international migration. But the most important factor explaining the differences is likely to be variations in age and ethnic compositions of the resident populations.

4.2  Variations in age structure

This overview of the role of migration in recent urban population change concludes with a brief assessment of differences in age structure amongst the internal immigrant, overseas immigrant and resident populations in New Zealand's urban and rural settlements. Variations in these structures further complicate any assessment of the role of migration in population change.

4.2.1  Internal and overseas immigrants compared

The migrants into the main urban areas from other parts of New Zealand tend to be much more heavily concentrated in the 15-24 age group (33 percent) than is the case for internal immigrant flows into secondary urban areas (17 percent), minor urban areas (17 percent), rural centres (16 percent) or rural areas (15 percent) (Table 4.2). The smaller cities and towns have their largest percentage of immigrants from elsewhere in New Zealand in the age group 25-34 years (20-22 percent, compared with 17 percent for MUAs). There are also higher proportions of internal immigrants in the 45-64 year age group in the smaller cities, minor urban areas and rural areas (18-20 percent) than is the case in the larger cities (14 percent) (Table 4.2).

Overseas immigrants, on the other hand, are most heavily concentrated in the 25-34 age group (32-37 percent) in all of the major settlement types, with the highest concentration in this age group found in rural areas (Table 4.2). The next most important age group in terms of percentage share of the immigrants from overseas is aged 35-44 years (19-21 percent in all settlement types).

Source: As for Table 2.1.

Internal immigrants and overseas immigrants, therefore, make differential contributions to the resident age structures of urban and rural populations. The internal migrants add significantly to the younger and older labour force age groups, while the overseas immigrants add to the middle aged population of working ages.

4.2.2  Differences between MUA

At the level of the individual city there are some significant variations. Dunedin and Palmerston North had the heaviest concentrations of their internal immigrants in the age group 15-24 years (53 percent and 46 percent respectively), largely because of in-migration of university students. Hamilton, Wellington and Christchurch (all with between 33 and 35 percent in this age group) also had an obvious "student bulge" in their immigrant populations. Auckland had a much less obvious student concentration; the share of internal migrants aged 15-24 years in this MUA (29 percent) was not much greater than the share for the age group 25-34 years (25 percent) (Table 4.3).