You decide Australia's population, we'll show you how it looks

Australia's population has quadrupled in the past century, with the number of people tipped to reach 25 million this year.

If current trends continue the population could top 40 million within 40 years, stretching hospitals, schools and transport if government spending doesn't keep pace.

On the other hand, it also means more people to produce goods and services, and to pay the taxes that fund government services.

Some say Australia should have stopped growing decades ago. Others point out Australia is a wealthy country with plenty of space to welcome more.

So we took a closer look at what different population sizes mean for Australia.

The chart below shows 24 potential paths for Australia's future population, based on the latest projections from the Australian Bureau of Statistics. Each assumes different levels of births, deaths and immigration — the key factors that determine population size.

If we set net overseas migration (the number of people moving to Australia minus those leaving) to zero in these projections, Australia's population shrinks in all but one scenario. These are the red lines you can see in the chart above.

The smallest projected population is just over 17 million people.

On the other hand, the biggest Australia we see is more than 70 million people — that scenario assumes a "high" net overseas migration, "high" fertility and "high" life expectancy.

But population size is just one piece of the puzzle — and it may not even be the most important, according to some experts.

"A lot of debates about population size seem to focus solely on the total population number. But in many ways the age structure of the population is much more important," demographer Tom Wilson, a principal research fellow at Charles Darwin University, said.

The age structure shows the number of people in each age group — in other words, the age breakdown of a population.

It's crucial to population planning and government spending because so many goods and services (think: schools, nursing homes, childcare and maternity services) are age-specific, according to Dr Wilson.

"In an ideal world, you want want to make sure the proportions of people in each age group don't vary wildly over time, so you don't have to, for example, build a whole load of schools and then shut down a whole load of schools," he said.

But global experience shows that's often easier said than done.

The demographic turning point Australia can't avoid

It is only a matter of time before the old outnumber the young.

Fertility rates have fallen steeply from a peak of 3.5 babies per woman in 1961 to 1.8 in 2016 -- below the level needed to replenish the population.

It's been below this level since 1976.

This, combined with people living longer, means Australia is rapidly approaching a future in which people over 64 outnumber those under 15 — an age balance that's "almost certainly unprecedented," Dr Wilson said.

"But whether it's a major problem remains to be seen… Many of those retirees are healthy and active — they're looking after children, travelling, maybe also working part time," he said.

"So often the population ageing narrative has quite negative associations… but in many ways it's actually a success story about combatting mortality."

Projections show Australia will reach this demographic turning point within four to 23 years, depending on rates of births, deaths or migration.

If both immigration and fertility increase, Australia won't hit this milestone until 2041.

But if fertility falls further and Australia has fewer immigrants than people leaving, the balance of old to young will start to reverse in 2022.

In this "zero net immigration" future, Australia will also achieve a second milestone: "peak child", the demographic moment when the number of children stops growing.

This would occur as early as next year if combined with low fertility, while high fertility would only delay this shift to 2025, according to ABS projections.

Bigger and younger, or older and smaller?

These charts shows how the Australia's age structure has changed over time.

Each line represents a year.

The shape of the line shows the proportions of old and young. A line with a peak or bump to the left means the young outnumber the old, while a peak or bump to the right shows the opposite.

Over the 20th century, Australia's population has become increasingly old. This is why the peak on the left of the charts below flatten out and the curve to the right swells as time passes.

In the above "zero net immigration and low fertility" example, you can see dramatic growth in the percentage of older people as the lines swell to the right.

When you compare it to a scenario with high immigration and high fertility, the increase in the percentage of older Australians is much less pronounced.

Ageing populations face a number of risks to living standards, including increased health and aged care costs, and a smaller share of working-age people to produce goods, services and wealth.

Perhaps most alarmingly, however, is the threat of a disappearing population.

In South Korea and Japan, for example, very low birth rates combined with few immigrants and high life expectancy have led to a dwindling workforce and rapidly-growing old-age population.

"Demographically these countries are in quite serious trouble," Dr Wilson said.

"Once a population has been shrinking for many decades, it's very difficult to get out of that trajectory, like trying to turn an oil supertanker."

At the same time, there are sharp differences in the population needs of regional and remote areas, or even of different capital cities, experts point out.

"It's also about the geographic distribution of the population, not just the total number," said Dianne Rudd, senior lecturer in geography, environment and population at the University of Adelaide.

"Yes there might be congestion in Sydney and Melbourne but in Adelaide, we're clamouring for migrants… so clearly there's regions that need more people and regions that would like less."

In some parts of Australia, immigrants don't just bring essential skills, Dr Rudd said. They're often the only demographic force battling population decline as an increasing number of Australians abandon the country for the cities.

"It's not just immigration that's causing uneven growth. Most Australians want to work in the big cities as well because that's where the opportunities are," Dr Rudd said.

"You can't just blame immigrants."

A rising economic burden

This chart shows the potential economic burden on the working population, measured by comparing the ratio of "dependents" (defined as those younger than 15 or older than 64) to those of working-age (between 15 and 64 years).

The higher the ratio, the higher the burden, since it means fewer working-age people to produce goods and services, and pay taxes.

As the last of the baby boomers retire, the economic burden will inevitably increase. But how fast depends largely on the number of overseas migrants.

The chart above plots Australia's future with medium levels of fertility, life expectancy and immigration.

In this scenario the number of dependents rises to 72 dependents per 100 working-age people in 2101.

The dependency ratio rises slowest in a future with high immigration, high fertility and high life expectancy, reaching 68 dependents per 100 working-age people in 2101, compared to roughly 53 per 100 today.

It leaps to a staggering 102 dependents per 100 working-age people in a future with zero immigration, low fertility and high life expectancy.

Throughout the 20th century, most of Australia's population growth came from natural increase. Natural increase contributed two-thirds of the rise in population between 1901 and 2001, according to the Home Affairs department.

But over the past decade net overseas migration has overtaken births as the main component of population growth. It has also been the biggest contributor to growth in Australia's working-age population since at least the 1980s.

"It's hard to see birth rates reverting to what they were in the '50s and '60s," ABS demographer Andrew Howe said.

"So if your birth rate isn't going to resurrect or sustain a healthy dependency ratio, then the other lever you've got is immigration."

Migrants boost economic growth by increasing the percentage of people in the workforce. They also tend to be younger, better educated and more motivated than the average resident "owing to their decision to move to Australia", according to the 2015 Intergenerational Report.

This is especially true in Australia, which has strongly encouraged skilled migration since the mid-1990s.

"If we didn't have as much migration not only would Australia be much smaller, it would be much older because migrants are traditionally younger than the average resident," Mr Howe said.

And because migrants are typically younger, they also tend to have children here. "So there's a multiplier effect with migrants not just boosting population numbers but also the fertility rate."

This is the reasoning behind Home Affairs Minister Peter Dutton's comments: "I want to bring people in as young as possible, as highly skilled as possible so they're paying taxes for longer, they're contributing to Australian society and they're helping build our nation."

It's high time the population debate moved beyond questions of taxes and spending, Dr Rudd said. For years, demographers have called for government policies to also consider social, cultural and environmental issues.

"That's the big picture that no one wants to talk about."

Ultimately, one population size won't fit all.

"You don't want workers paying huge amounts to support other sections of the population ... but there's no magic number," Dr Wilson said.

"It all depends on your perspective — whether you want to see economic expansion, for example, or prioritise protection of the environment."

Credits

Data and reporting: Inga Ting

Digital production and reporting: Mark Doman

Development: Ri Liu, Nathan Hoad and Nathanael Scott

Design: Alex Palmer

Some notes about this story:

Data for 1911-2011 are from the Australian Bureau of Statistics Australian Historical Population Statistics, 2014.

Data for ages 85-59 for the years 1911-2011 in the Population Age Structure chart include people aged 85 and older.

Data for 2012-2101 are based on Australian Bureau of Statistics Population Projections, released in 2013, so population figures for 2012-2017 are estimates.

High fertility assumes a total fertility rate of 2.0 babies per woman; medium fertility assumes 1.8 babies per woman; low fertility assumes 1.6 babies per woman.

High life expectancy assumes continued improvement to life expectancy, with male life expectancy reaching 92.1 years and female life expectancy reaching 93.6 years in 2060-61.

Medium life expectancy assumes declining improvement to life expectancy, with male life expectancy at birth expected to reach 85.2 years and female life expectancy to reach 88.3 years in 2060-61.

High net overseas migration (NOM) assumes 280,000 people per year; medium NOM assumes 240,000 people per year; low NOM assumes 200,000 people per year.

More information on these assumptions is available on the population projection section of the Australian Bureau of Statistics site.

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