Melbourne surpasses Sydney as Australia's most populous city, on technicality: All you need to know
The difference in population, however, is less than 19,000. Sydney had been Australia's most populous city since 1902, when it overtook Melbourne.
Melbourne, Australia's cultural capital, has finally overtaken Sydney as the country's most-populous city, doing so on the basis of a ‘technicality.’ According to the Sydney Morning Herald, a 'boundary change' for Melbourne made it Australia's largest city, as it swapped places with its arch-rival.

Here is all you need to know:
(1.) Melton, which lies on the northwestern fringe of Melbourne, was included in the latter's Significant Urban Area, lifting the metropolis' population to 4,875,390 (June 2021).
(2.) On the other hand, Sydney, which held the top spot since 1902, had 4,856,693 residents as of June 2021. This, therefore, means Melbourne had just 18,697 more residents than Sydney.
(3.) The Greater Sydney area, however, continues to have more people than its Melbourne equivalent: 5.26 million, 4.98 million (June 2021). In January this year, however, the Australian government projected Greater Melbourne to have more people than the Sydney one, by 2031-32.
(4.) At the turn of the current century, Greater Sydney's population was 17% more than that of Greater Melbourne. The gap, however, had been narrowing down since, and was at 10% and 6% in 2011 and 2016, respectively.
(5.) According to the Australian Bureau of Statistics (ABS), the island-continent's current population is 26 million. New South Wales (NSW), of which Sydney is the capital, continues to be the most populous province, ahead of Victoria, which has Melbourne as its capital. ABS will release the latest population estimates on April 20.