That seems like a reasonable estimate to me. But does it take into account how much the government is willing to sacrifice the environment for extra population? Extra water is easy to obtain through desalination, assuming one has the energy for it--Australia has much coal to produce energy, and for renewable energy, has bountiful solar potential. There's also uranium mining, since Australia has massive uranium reserves.
The biggest thing is that with extra population, Australia's food exports will decline because of the need to feed its own population.
if you are going that way
129.57 million unemployed, immobile, probably homeless and generally unhappy people.
There are lots of factors which need to be considered in asking a question like this, but I’m assuming that the question means to ask about the maximum population Australia could sustain on its own resources.
If that’s the case, then Australia's sustainability runs into trouble immediately, as the country
is nearly completely dependent on imported fuel. The report in that linked article estimated that
91 percent of Australia's oil comes from foreign sources.
Australia produces in the neighborhood of 330,000 barrels of oil per day on its own, while
it consumes about 1,080,000 barrels of oil per day. About three-fourths of that consumption is used for transportation.
Without doing a lot of number crunching, it's pretty easy to see that constraining Australia's economy to run on the energy it produces on its own - even when accounting for alternative sources of energy - would severely restrict Australia's ability to sustain a large, interconnected population.
Before we explore that in more detail, let's look at what energy restrictions would mean for the most vital component of population sustainability: food. Despite
94 percent of Australia's land area being, well,
shit for farming† (and about 10.5 percent of it is protected from development of any kind), the country does have a significant agricultural sector.
Source: Australia's Department of Foreign Affairs and Trade, "Feeding the Future."
As a percentage of GDP, Australia's agricultural sector contributes
about twice as much to its economy as the US' sector (although nowhere near as much in terms of volume). Most of this revenue comes from exports, as
60 percent of food grown in Australia is sent overseas. On paper, then, Australia could more than double the amount of food available to its population based on current production.
But let's go back to the oil problem.
In 2012-2013, Australia's agricultural sector used 90.4 PJ of energy from refined petroleum products‡. That is just shy of 15.4 million barrels of oil equivalent, or 42,500 barrels of oil equivalent per day. Of course, those are
refined products' barrel equivalent, which means the actual deficiency is even greater (ie, it doesn't account for the energy needed to convert the crude product into refined gasoline). Still, it's well below the average daily oil production in Australia.
Keep in mind, though, the operation of farms is dependent on the energy sector; and while Australia's energy sector is 98 percent dependent on coal and natural gas (77 and 21 percent, respectfully), the mining sector which provides the coal for power plants gets just less than half of its energy needs from oil products, and it consumes more than twice as much oil equivalent as agriculture at just under 100,000 boe per day.
Saving the mining industry would keep Australia's power plants running, as
75 percent of electricity in Australia is generated from coal, which would allow farms to continue operating. The issue becomes figuring out how much fuel is left to transport food to the Australian people.
Taking out mining and agriculture, Australia is down to producing 187,500 barrels of oil per day. This is a problem, because Australia consumes an average of 530,000 barrels of oil equivalent per day for transportation - almost three times as much as what we've made available.
In this scenario, the Australian government would almost certainly have to restrict fuel consumption to commercial vehicles. But would that be enough?
The restrictions would go a long way, but there would still be problems. Passenger vehicles account for
approximately 58 percent of vehicle fuel consumption in Australia, but that means commercial vehicles consume about 220,000 barrels of oil equivalent per day - still more than what would be available.
Of course, not all freight they carry is food-related. According to a 2002 study by the Australian Bureau of Statistics,
just shy of 30 percent of road freight in 2000 was for the transportation of food and beverage. If that level has remained constant, then that gives us about 66,000 boe used for the transporting of food. As a result, Australia would have a surplus of 121,500 barrels of oil daily production to use in other sectors. 20,000 of those barrels (equivalent), however, would have to be used to keep trains running (which, not incidentally, carry just shy of 14 percent of Australians' food), leaving a grand total of 101,500 boe/day left for general economic use.
That wouldn't be nearly enough to sustain Australia's current economy, much less one to support a population that approached the maximum "sustainable" level, but we're not focused on that for this question.
If 40 percent of Australia's agricultural products are going towards feeding 23.71 million people, then the entirety of Australia's food production could feed 59.28 million (which jives with data from the National Farmers Federation of Australia, which states that Australia produces enough food to feed 60 million people).
According to the
Food and Agriculture Organization of the United Nations, the average Australian consumes about 3,276 calories per day. Let's say for this hypothetical, though, that on top of severely rationing fuel, the Australian government also rations people's food consumption to the bare minimum (
1,200 calories for women, 1800 for men, which comes out to a population-wide average caloric intake of 1,500/person), then the amount of food produced in Australia could feed 129.57 million people.
I have no idea how any of them would be able to afford to eat, what with unemployment through the roof due to the decimated economy, or how they'd get to food centers with the abolishing of most modes of personal transportation, but the food would be there for them if they could manage it.
Fortunately, finding room for all these people would not be a problem. Australia, at about the size of the continental US (population: 310 million), would have plenty of room to grow if Australians decided that land preservation was no longer a thing they wanted to care about.
However, if Australia's urban planners took a look at the uselessness of urban sprawl in a country where nobody can travel and decided to compact everyone into mega-dense cities, that
also might not be a problem.
89 percent of Australians already live in urban areas (see: a lot of the land, while beautiful, is shit for living), making it one of the most highly urbanized populations on the planet. So while it's often cited that Australia, on the whole, is one of the least most densely populated countries on the planet, when it comes to the cities, it has an average population density of 1,097 people/km2'>km 2 km2 , which is about the population density of Phoenix, Arizona.
If on top of the food and fuel rationing, the Australian government also froze the amount of land available for urban development into which it crammed all non-farm laborers (eg, effectively everyone), then the final, average urban population density would be about 6,737 people/km2'>km 2 km2 , or somewhere between the population densities of Kyoto and Rio de Janeiro.
Again, I have no idea how people would afford to live in these cities, as housing prices would skyrocket - assuming the homes were even built, what with a dramatically reduced construction and manufacturing sector - but the point is that they'd all (uncomfortably) fit.
† Australian Department of Foreign Affairs and Trade, "
Australia's environment at a glance":
Only six per cent of the Australian landmass is arable. Large volumes of water are required from both surface and groundwater supplies. Australian soils are highly dependent upon vegetation cover to generate nutrients and for stability. Land clearing, water extraction and poor soil conservation are all causes of a decline in the quality of Australia’s soils.
‡
Australian Bureau of Energy and Tourism, Office of the Chief Economist, "
2014 Australian energy statistics data" Table F.
https://www.quora.com/What-is-the-highest-possible-population-that-Australia-could-sustain
Basically even technology its the land that is shit for farming, food will be a major issue