A study by Trubka, Newman and Bilsborough showed that each additional household requires infrastructure spending of over half a million dollars if services for existing households are not to be degraded. This means over $200,000 per additional person is required to maintain the quality of services such as roads, hospitals, schools, water supply, sewerage and the like. Since this money has not been spent (and we cannot afford it in future) any further population growth means that the quality of life of existing residents will decline.
Nevertheless, additional people from nations poorer than Australia think that our services are superior to what they are used to. Their lives are improved (at the cost of the lifestyles of the existing residents) and this is supported by a 2006 study made by the Productivity Commission They found that,
"the real average annual incomes of existing resident workers grows more slowly ... as additional immigrants place downward pressure on real wages."
So why can we not afford the infrastructure spending required for additional people? Consider that, on average, most infrastructure has about a 50-year lifespan before it must be replaced. This means that, if the population is stable, about 2% of infrastructure must be replaced each year. But if our population is currently growing at 1.7% (and in 2010 it was growing at 2.1%) then this means that we must also build nearly 2% of additional new infrastructure every year. So, with a growth rate of 1.7% our infrastructure costs are nearly doubled! The result is declining services, maintenance backlogs, and governments being forced to sell off public assets to try to find the money to pay for the extra people. Queensland's recent history is a prime example.
We often hear that Australia is huge compared to other much more crowded nations "so of course we can take more people". But Australia is also the driest continent on Earth with ancient soils depleted of nutrients that would be unable to produce the grain they do without large inputs of fertilizers and energy from fossil fuels (for tilling/harvesting etc.). In an average year we currently produce sufficient grain to feed 80 million vegetarians but in a drought year (such as 2006/7) we only grow 1.6x as much grain as we use. That means that, if we increased our population to 37 million we could not feed ourselves in a drought year. And as oil becomes more expensive and scarce in future (see below) we will have difficulty maintaining even current rates of food production. We are already not self sufficient in fruit and vegetables! These facts are outlined in an essay, Can we feed a 'Big Australia'?
We already see that many Australian cities have outgrown their natural water supplies and are beginning to rely on energy-intensive desalination to provide water in dry times. More people in our cities can only make this situation worse and will lead to permanent water restrictions. Now they are talking about recycling sewerage to provide drinking water. Currently our population is growing so fast it can fill a city the size of Canberra every year!
Oil is the world's most important energy source. Many Australian cities are now so large they could never be fed and serviced without oil. But the ability of the world to extract oil from the Earth has flat-lined leading to continuously high prices and faltering economic growth in the developed world. There are claims that these problems will be solved by production of "shale oil" but a comprehensive study has shown this to be completely inaccurate hype. Australia passed its peak of oil production in 2000 and may produce very little oil by the end of this decade. We are now very dependent on imported oil products such as petrol, diesel and aviation fuels. Most of this comes from refineries in Singapore. At the same time we are failing to hold the OECD-mandated 90 days of fuel in stocks and this makes us extremely vulnerable to any disruption of fuel supplies from Singapore that would certainly happen if, for example, there was a conflict in the Middle East that disrupted shipments of crude oil to Singapore's refineries. Increasing our population increases our food and fuel needs and simply makes us more and more vulnerable to shortages of these essential supplies.
The constantly increasing demand of more humans for more resources and more housing means that our natural environment is under relentless pressure. A 2010 report produced by Flinders University academics saw negative consequences for our environment and society from any further expansion of our population. (The initially released and very pessimistic report was quickly "corrected" [toned down] and rereleased by the Department of Immigration and Citizenship). Although some people may believe our continent is under-populated the falsity of this idea is shown by our impact on the rest of nature - Australia now has more species of animal in danger of extinction than any other continent.
There are many other indicators of the failing health of our environment due to an increasing human population. One that many people can understand is the declining availability of good sports fishing opportunities. Unfortunately, people living in cities often live in artificially "fertile" environments supported by oil, fertilizer, piped water and air conditioning so they do not notice the slow but relentless decay of the natural world that ultimately supports us.