Sydney has proudly held the title for more than 100 years.
But with populations rapidly growing on Melbourne's fringe, the city limits have been expanded to include the area of Melton.
The latest government figures, from June 2021, put Melbourne's population at 4,875,400 - 18,700 more than Sydney.
The Australian Bureau of Statistics (ABS) defines a city's "significant urban area", by including all connecting suburbs with more than 10,000 people.