Sydney has a bigger population than Melbourne, right? Well, maybe. A lot depends on where you draw Sydney's northern boundary. At the moment the official definition of Greater Sydney stretches all the way to Lake Macquarie, about 120 kilometres north of the CBD. That means the city's population is bolstered by the inclusion of the heavily populated NSW Central Coast. That region rates as Australia's ninth largest "significant urban area" in its own right, according to the Bureau of Statistics. It comfortably ranks above Wollongong, Hobart, Townsville and Darwin.

So how would Australia's two biggest cities compare if Sydney did not include the Central Coast? The bureau's latest estimates put the population of Greater Sydney at just over 5 million in June 2016. Greater Melbourne's head count stood at 4.73 million.

But if you remove the Central Coast's 335,000 residents from Sydney's tally it is a different story. That drags the harbour city's population back to 4.7 million – about 25,000 fewer than Greater Melbourne.

On that definition, Melbourne became Australia's biggest city in September 2015. And Greater Melbourne's population could be even bigger if the boundaries were tweaked a little.