Australia's population will reach an estimated 23 million people some time tonight.
The Australian Bureau of Statistics says the projection is based on last year's population estimate and takes into account factors such as the country's birth rate, death rate and international migration.
It estimates that with a birth every one minute and 44 seconds, a new migrant arriving every two minutes and 19 seconds, and a death every three minutes and 32 seconds, the 23 million mark will be reached just after 10:00pm (AEST).
With births outnumbering deaths two to one and a 14 per cent increase in migration, Australia's population is now growing by more than 1,000 people per day.
ABS figures show that around 180,000 people move to Australia each year.
The ABS estimates that if current migration and birth rates do not change, Australia's population will be 35 million in 2056, and 44 million in 2101.
But demographer Mark McCrindle says the growth in the population is being driven by net migration, and he says the 40 million milestone will come earlier than the ABS estimates.
"The proportion of the net migration to our population growth has increased from 54 per cent a year ago to now 60 per cent of our growth," he said.
"We can say that Australia has world-beating population growth right now. The world is growing 1.1 per cent per annum and Australia [at] 1.7 per cent is really out in front.
"When you compare us to comparable developed nations, we really are ahead of the comparisons."
And he says the population is on track to hit 40 million by the middle of the century.
"If the growth slows down a little bit, that might be in the 2050s, if it remains at current trend it will probably be in the early 2050s, but certainly that figure of 40 million is what we’re headed towards," he said.
"It’s inevitable that we will hit that number."
ABS director of demography Bjorn Davis says it is impossible to know who the 23 millionth person will be,.
"While we would like to think that the 23 millionth Australian would be a beautiful little baby boy, it could also be a British backpacker who's decided to stay in Australia and work in Australia. It could be an Australian who's been living overseas coming back to Australia, it could be a New Zealand citizen who's decided they want to live in Australia for a while," he said.
"The reality is the population clock gives us an answer to that question of how many people there are in Australia now, but it's based on taking our most recent estimate of the population that we released a few weeks ago for September 2012, and then making some assumptions.
"We certainly don't have people standing by with clipboards at the hospital watching the babies being born or people standing by at the airports with a bean counter counting how many people are crossing the border."