In 1820, the islands of present-day New Zealand had a population of approximately 100,000 people. This figure would fall until the early 1840s, partly as a result of European diseases brought by colonizers, and a series of destructive inter-tribal wars among the Māori peoples. These conflicts were named the Musket Wars due to the European weapons whose introduction instigated the conflicts, and the wars saw the deaths of between 20,000 and 40,000 Māori, from 1807 to 1837. After falling to just 82 thousand in the 1840s, the population would begin to rise again in 1841 following the establishment of New Zealand as an official British colony, with a strong promotion of European settlement by British citizens sponsored by the Church of England. European migration to New Zealand was low in these early decades, but increased in the mid-19th century, particularly following the discovery of gold in New Zealand’s South Island in the 1860s. This growth would continue throughout the 1870s, in part the result of a strong promotion of mass migration from Britain by Premier Julius Vogel’s administration.

Early 20th century However, between 1881 and the 1920s, the New Zealand government heavily restricted Asiatic migration to the islands, resulting in a fall of population growth rate, which would remain until the Second World War. The country would experience a dip in population during the First World War, in which New Zealand would suffer approximately 18,000 military fatalities, and another 9,000 lost to the coinciding Spanish Flu epidemic. The population would stagnate again in the Second World War, which resulted in the death of almost 12,000 New Zealanders. In the years following the war, New Zealand would see a significant increase in population due to the mixture of a baby boom and a migrant spike from Europe and Asia, following a large demand for unskilled labor. Recent decades This increase continued for several decades, until international factors, such as the oil crises of 1973 and 1979, and the UK's accession to the European Economic Communities (which ended most of New Zealand's trade agreements with Britain; it's largest trade partner), greatly weakened New Zealand's economy in the 1970s. As a result, population growth stagnated during the 1970s, while economic problems persisted into the early 2000s. In contrast, the Great Recession of 2008 did not impact New Zealand as severely as most other developed nations, which allowed the economy to emerge as one of the fastest growing in the world, also leading to dropped unemployment levels and increased living standards. In 2020, with a population of almost five million people, New Zealand is regarded as one of the top countries in the world in terms of human development, quality of life and social freedoms.

Life expectancy in New Zealand was just over 34 in the year 1870, and over the course of the next 150 years, it is expected to have increased to just over 82 by the year 2020. Although life expectancy has generally increased along a gradual curve throughout New Zealand's history, there was a slight dip in the late 1910s, as a result of the First World War and following influenza epidemic. Apart from this, there were no anomalies that reduced life expectancy throughout this 150 year period.

The influenza pandemic of 1918, known as the Spanish Flu, was one of the deadliest and widespread pandemics in human history. The scale of the outbreak, as well as limitations in technology, medicine and communication, create difficulties when trying to uncover accurate figures relating to the pandemic. Estimates suggest that the virus, known as the H1N1 influenza virus, infected more than one quarter of the global population, which equated to approximately 500 million people in 1920. It was responsible for roughly 25 million fatalities, although some projections suggest that it could have caused double this number of deaths. The exact origins of this strain of influenza remain unclear to this day, however it was first noticed in Western Europe in the latter stages of the First World War. Wartime censorship in Europe meant that the severity of the pandemic was under-reported, while news outlets in neutral Spain were free to report openly about the impact of the virus; this gave the illusion that the virus was particularly strong in Spain, giving way to the term "Spanish Flu".

Effects of the virus

By late summer 1918, the pandemic had spread across the entire continent, and the H1N1 virus had mutated into a deadlier strain that weakened the infected's immune system more than traditional influenzas. Some studies suggest that, in contrast to these traditional influenza viruses, having a stronger immune system was actually a liability in the case of the H1N1 virus as it triggered what is known as a "cytokine storm". This is where white blood cells release proteins called cytokines, which signal the body to attack the virus, in turn releasing more white blood cells which release more cytokines. This cycle over-works and greatly weakens the immune system, often giving way to other infections; most commonly pneumonia in the case of the Spanish Flu. For this reason, the Spanish Flu had an uncommonly high fatality rate among young adults, who are traditionally the healthiest group in society. Some theories for the disproportionate death-rate among young adults suggest that the elderly's immune systems benefitted from exposure to earlier influenza pandemics, such as the "Asiatic/Russian Flu" pandemic of 1889.

Decrease in life expectancy As the war in Europe came to an end, soldiers returning home brought the disease to all corners of the world, and the pandemic reached global proportions. Isolated and under-developed nations were especially vulnerable; particularly in Samoa, where almost one quarter of the population died within two months and life expectancy fell to just barely over one year for those born in 1918; this was due to the arrival of a passenger ship from New Zealand in November 1918, where the infected passengers were not quarantined on board, allowing the disease to spread rapidly. Other areas where life expectancy dropped below ten years for those born in 1918 were present-day Afghanistan, the Congo, Fiji, Guatemala, Kenya, Micronesia, Serbia, Tonga and Uganda. The British Raj, now Bangladesh, India and Pakistan, saw more fatalities than any other region, with as many as five percent of the entire population perishing as a result of the pandemic. The pandemic also had a high fatality rate among pregnant women and infants, and greatly impacted infant mortality rates across the world. There were several waves of the pandemic until late 1920, although they decreased in severity as time progressed, and none were as fatal as the outbreak in 1918. A new strain of the H1N1 influenza virus did re-emerge in 2009, and was colloquially known as "Swine Flu"; thankfully it had a much lower fatality rate due to medical advancements across the twentieth century.