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.

Melbourne is on track to become the nation’s biggest city Darrian Traynor

The adjacent Geelong region, for example, is not included in Melbourne's population count. And yet Geelong is a little closer to Melbourne's CBD than the central coast hub of Gosford is to downtown Sydney.

So if Greater Sydney did not include the Central Coast and Greater Melbourne did include Geelong, the Victorian capital would be Australia's biggest city by a significant margin.

The Bureau of Statistics includes the Central Coast as a part of Greater Sydney because of the relatively large share of people from that region who commute south for work and study. The 2011 Census counted 23,100 from the Central Coast who travelled to the rest of Greater Sydney to work – about 7 per cent of the total population. Even so, it is still debatable whether that region – which goes way beyond the traditional "Sydney basin" – should be considered part of the harbour city.

Tim Williams, the chief executive of business lobby group the Committee for Sydney, says Sydneysiders themselves aren't too sure if the Central Coast is part of the city. "Sometimes we seem to include it and sometime we don't," he said. "I worry that we do include it sometimes to boost the numbers."

The scope of the powerful Greater Sydney Commission set up recently by the NSW government to "coordinate and align" the planning is also telling. It doesn't include the Central Coast.

No matter where you think Greater Sydney's boundaries should be drawn, this comparison draws attention to a striking demographic trend - Melbourne's burgeoning growth.

The population of the Victorian capital grew by nearly a million people during the past decade - almost 200,000 more than Sydney added in the same period. That rapid growth has been driven by a stream of arrivals from other Australian states, including NSW, as well as net overseas migration.

Melbourne is on track to become the nation's biggest city, regardless of whether the Central Coast continues to bolster the numbers for Greater Sydney.