Collectively, it's a happy moment to welcome our 25 millionth person. But it still raises big questions about Big Australia. Our population is small compared to other nations in part because vast tracts of the continent are presently uninhabitable, climate change suggests it'll become worse; for example the Northern Territory going from 11 to 308 days above 35 degrees Celsius by 2070.

At the same time the push to develop Australia's north is in preparation for a population of close to 50 million by 2075. Is there wisdom in building new cities in places that might be less inhabitable? By 2040 one in five Australians will be over the age of 65, putting even greater pressure on younger generations than we have today.

Yes, we can push the working age upwards but to what degree does that then further compound the related issue we have of younger unemployment rates, which then impedes the replacement rate and again deepens the problem of the dependency ratio longer term - a reciprocal spiral.

At present our lifestyle is reasonably good and our democracy stronger than others. But our economy needs work. Those in support of Big Australia believe we need to grow the population to support economic growth but that's entirely predicated on an economy that is genuinely productive, capable of surplus trade, and has the capacity to feed, water, shelter and clothes its own citizens.

Right now Australian farmers are crying out for us to listen to the warning signs about our own carrying capacity. Just how many people can Australia support?  Our carbon emissions and ecological footprint are among the biggest in the world; World Overshoot Day came around last week and Australians are still living as if we had three or more planets worth of resources. 

This means our lifestyle is already unsustainable long before we add more people. At a household level, studies show Chinese migrants to Australia triple average Australian consumption in their first few years of residency, a kind of adaptive overshoot that settles down as they become more acculturated. I suspect the same happens with many other ethnicities but collectively we need to consume resources that more directly support shared health and wellbeing. 

Also, the more unequal we become as a society the more population growth serves the big end of town rather than its citizens. This is simply because what looks like growth in the national accounts is becoming increasingly enjoyed by fewer Australians - wealth is becoming obscenely concentrated at a global level. Look at the $444 million given to a private foundation for the Barrier Reef this week.

How much will be spent on junkets and lobbying supporting vested interests? Which raises another mythology on economic growth - counting the ecological or human costs of cleaning up the externalities of economic growth as actual growth itself. Selling cigarettes and then treating Chronic Obstructive Pulmonary Disease are counted as economic growth!

Had we protected the Reef in the first place, that $444 million wouldn't be what amounts to a maintenance cost and could have gone to genuine progress, for example health, education and childcare in Indigenous communities, maybe housing the homeless, or helping working mums and families.

Which raises another issue. If we need to mitigate climate change by changing lifestyle then the current state of play will lead to lower Australian life expectancies just as climate change kicks in with more ferocity - a double whammy from the profligate consumption of the past few decades that preferentially served wealthier and older Australians.

Can we carry more people with an aging population, little or no surplus GDP other than that artificially created by construction for a growing population, dwindling export of education and a mining industry that's predicated on further emissions? I would love to say welcome to country. 

But let's make sure it remains a nation worth joining for the long-term, and that needs some political leadership to balance some mighty big issues.

From ecological perspective our consumption is the main culprit but not the population in numbers

Debates against population growth in Australia tend to ignore the fact that Australia’s current ecological footprint is 9.3 global hectares per person, which is the fourth highest in the world.  Therefore, it is not population growth per se but Australia’s unsustainable consumption is what needs to be checked. Considering a hypothetical life span of one year for the world, if every person in the world consumed like the average Australian then the world would have overshot its ecological capacity by 31 March. At the same time if every person in the world consumed like the average Vietnamese (which has a total population almost four times higher than Australia) world would overshoot its ecological capacity only by 21 December.

Population redistribution should be the prime focus rather the population growth as a whole

Australia’s population is currently distributed unevenly among the states/territories. There should be ways and means to ensure the overseas migrants settle in states such as South Australia, whose Premier is advocating a higher population growth for the state’s economy. There is also a danger that states like South Australia can end up with fewer f highly skilled females in the prime economically productive ages. In recent years South Australia experienced the largest net loss of females   in the prime working ages from 20 to 34 years to interstate destinations.  Moreover, females younger than 20 to migrating to other states are predominantly unmarried new job seekers who have mostly completed their education and are seeking wider opportunities and excitement offered by the larger and more diverse job markets and higher wages in the eastern cities of Melbourne, Sydney and Brisbane. Debates should focus on how to stem the flow of female interstate migrants from a state like South Australia.       

Australia is the worlds 6th largest country by area, yet, according to the UN, we are 53rd in terms of population, close to that of Madagascar and North Korea, both countries being just a fraction of our geographical area. With the population of the world continuing to grow exponentially, there is no likelihood of a decrease in  human pressure on productive resources, and if Australia is to prosper in a sustainable future, we must recognise that this huge land area will have to provide a home for many more people in the future.

The question we must consider today is what degree of choice do we want to have over how this increase in population will be catered for. We also need to recognise that people provide both the labour force for our economy, and the market for our goods. Since diversity is a key driver of innovation, we should be celebrating this milestone as a stepping stone to the future success of Australia in an ever-changing world. In order to make the most of this situation,  what we need to do now is make sure we have the necessary housing, health, education and resource extraction policies in place to cope with these future changes.

It is essential to rethink how aged care is provided and supported. Aged care nurses are the Cinderella of the nursing workforce – lower paid, lower status, excluded from quality criteria such as ratios. Yet how will Australia fare without well-educated, innovative nurses to support our growing and ageing population?

While planners in Australia’s capital cities are designing strategies to cope with population growth, those in regional cities and towns are designing strategies to promote growth. As Australia’s population moves past the 25,000,000 mark, serious consideration needs to be given to population distribution. In 2-4 years, Australia will add another 1 million people to the population. Increasing the population density of Australia’s capital cities to accommodate this population growth can only take us so far. It is time to examine why so many of us are based in major cities and how Australia’s regional cities and town can be supported to become a viable alternative.

For many, living in regional cities and town is not viewed as a viable option due to limited employment, education, health care and social support options. The long-term disinvestment in regional and rural communities by successive governments has made it all the more difficult for these places to diversity their economic activities, grow employment opportunities and attract new residents.

Australia does not have a national population strategy. Many individual towns and cities across the country are trying, best they can, to cope with population growth, stimulate population growth, or, for some, curb population loss. Without a national framework to bring this knowledge and effort together, opportunities are being missed.  Now is the time to get serious about a national population strategy

"The question of population in Australia is complex, but simple in the required intent. We will be faced with massive pressures to admit not a few, but millions of people of great cultural diversity with accelerating climate change over the next 10-20 years.

The Chinese Academy of Sciences estimates the displacement in the reaches of the disappearing Tibetan-sourced river system to be of the order of 2.3 billion people.

Hansen et al from Columbia University predict millions to be affected in coastal areas with ice melts and major storms.

Steffen et al from ANU, Stockholm and other institutions have published in PNAS that global warming is near the tipping point, into a hothouse earth affecting populations at large.

We need visionary, problem-solving leadership and community action for aggressive family planning, ecosystem management, socioeconomic approaches without consumerism and waste, and the cessation of use of fossil fuels, all in crisis mode.

In the meantime, we can respectfully and supportively admit displaced persons and well-intentioned migrants in accordance with our culturally pluralistic principles.

As we do so, we must invest in decentralising infrastructure with minimisation of ecosystem loss."

Population size is not the primary issue for Australia, it's how we live, the resources we use, the waste we create and the way we move around. Australians have some of the largest per capita ecological footprints globally and we need to dramatically reduce them.

Better planning across the boards is needed to enable Australians to be less dependent on fossil fuels, especially for transport, cooling and heating. We have only just begun to tap the vast supply of renewable energy available to us. International experience shows how we can provide choices for people to walk, cycle or take public transport at double or triple the current numbers.

Australia grows and produces an increasingly wide variety of food, and the vast majority is exported to feed populations several times our own size. We also import many things we could grow or make in Australia. As our population grows, we could not only feed more Australians but also reduce our ecological footprint by reducing imports and cutting exports of products and raw materials, like coal which is of little benefit to Australia but a major contributor to human-induced climate change.

Population is a key factor that determines our extent of 'Ecological Overshoot'. 

We are using more Earths than we can afford, and it is crucial that we look for smarter and more sustainable ways of designing cities - in particular regarding infrastructure and our food and energy systems.