If we are going to have great debate about whether we want a Big Australia, people will need a much stronger grasp on the factors driving population growth and immigration than they've shown so far.
This is the rationale for a useful booklet, Population and Immigration: Understanding the Numbers, issued by the Productivity Commission this week.
Over the past 50 years, Australia's population has averaged growth of 1.6 per cent a year, causing it to double to 22.3 million. This is faster than for most developed countries.
The growth in our population comes from two factors: natural increase (more births than deaths) and "net overseas migration" (more immigrants than emigrants).
Natural increase is relatively stable, averaging about 130,000 people a year, whereas net migration can vary a lot from year to year.
Our "total fertility rate" (the number of babies per woman) has risen a bit in recent years to 1.9, although it's only about half the peak it reached in the 1960s.
It's fallen over the decades because of more effective contraception, the higher education of girls, and married women wanting to return to the paid workforce.
It's recovered a bit in recent years because of a slight reversal of the trend for women to leave starting their families later and later. Women worry more about leaving it too late and, when they start a bit earlier, more couples are able to achieve the common desire to have two kids rather than one.
The commission doubts whether Peter Costello's baby bonus has had any significant effect on fertility.
Demographers put the population "replacement rate" at 2.1 children (the extra 0.1 is to allow for a few who die before being able to reproduce). Since our fertility rate has long been below that (as it is in most developed countries), without net migration our population eventually would start to fall.
However, natural increase has been kept positive by rising longevity (a falling death rate). Longevity has risen significantly over the past century because of improvements in public health measures, improved nutrition (from a rising material standard of living) and advances in medical science.
Since the 1980s, net migration has overtaken natural increase as the main contributor to population growth. In the 1970s it accounted for about 30 per cent of population growth but in the past 10 years it's grown strongly to now account for about 65 per cent of the growth.
Our long-term rate of population growth is 1.6 per cent but in recent years strong migration has caused growth to be higher than that, with a rise of 2 per cent in the year to June 2009. This included net migration for the year of 313,500.
This high level of migration - combined with Treasury's projection that our population could reach 36 million by 2050, Kevin Rudd's remark that he believed in a big Australia and public anxiety over boat people - has prompted the debate about Big Australia.
But there's a lot of confusion over the extent to which the government controls the level of immigration.
Immigrants can be divided into two streams: those coming permanently and those coming temporarily. Starting with the former, the government has a permanent migration program. Each year it decides on the maximum number of permanent immigrants it will take and this figure gets a lot of publicity.
The limits set for this financial year are unchanged from last year: a total of almost 169,000, being 114,000 places for skilled migration plus 55,000 places for families. The big increase in recent years has been in the skilled category.
Also in the permanent stream is the government's humanitarian program. Each year the government sets a limit of about 14,000 on the number of refugees it's prepared to let in. People who arrive by boat and are found to be genuine refugees are given permanent residence under this program.
But fewer than 3000 humanitarian places a year (less than 20 per cent) are given to people who apply after they get here. The rest apply overseas and the program doesn't increase to make room for onshore applicants.
So repeated TV footage of people arriving on overcrowded boats has left the public with a quite exaggerated impression of how many of them there are. Some people imagine it's boat people who explain the high levels of migration in recent years but that's quite wrong. Their numbers are trivial in the scheme of things and don't increase the modest total of refugees admitted each year.
Finally in the permanent stream come Kiwis. Just as you and I can move to New Zealand any time we choose, so Kiwis can come here without government permission.
But here's the trick: most of the growth in net migration in recent times has been in the short-term stream, accounting for about two-thirds of annual net migration. In June 2009, there was a stock of almost a million people in the country on temporary visas. The three main temporary categories are: overseas students (contributing 110,000 to net migration in 2007-08); long-stay "457" business visas (contributing about 35,000); and working-holiday visas (about 21,000).
Long-stay business visas can run for as long as four years. In principle, if there was no increase in the number of people in these three categories over time, they'd make no contribution to population growth. About the same number of people would be coming and going each year.
Similarly, if that was all there was to it, any increase in their numbers would make only a temporary contribution to population growth. Eventually the increase would stop and eventually they'd go back home.
But in recent years more than half the people with long-stay business visas have been granted permanent residency, as have about a third of the overseas students.
Now, it's important to realise the government imposes no limits on any of these categories. Overseas student numbers are driven by the efforts of Australian universities and private training colleges to attract paying customers. The long-stay business visa numbers are driven by employer demand for skilled workers not available locally.
But the government has recently more than halved its list of skilled occupations in short supply and tightened up on the overseas student category. Combine this with the high dollar and the troubles of Indian students in Melbourne and it seems likely the number of overseas students will now fall quite heavily.
It's a safe bet net migration won't grow nearly as fast in the next few years.
