Melbourne and Sydney: How do you work out the population of a city?

For more than a century Sydney was the biggest city in Australia when it came to the population - but it's now lost that title, thanks to change in how the figures are counted.

Last week Melbourne officially overtook it as Australia's biggest city with a population 4,875,400 at the country's most recent census in 2021.

A census is when the government ask people to share lots of information about themselves, so they can have a better idea of who is living in the country.

According to data from the Australian Bureau of Statistics (ABS) Sydney had 18,700 fewer people living within the city limits than Melbourne.

Sydney can be found on Australia's east coast in the state of New South Wales, and is known for its magnificent harbour and famous landmarks like the Sydney Opera House and Harbour Bridge.

Melbourne is in the south-eastern part of mainland Australia, within the state of Victoria. It's not as well-known as Sydney but also attracts many tourists every year.

One of the main methods the ABS uses to count population is using something called "significant urban area" measure - which defines somewhere as being part of a city boundary if it has an urban population above 10,000.

This meant that when the ABS were processing the census data, they changed the boundary it used previously to calculate Melbourne's population.

As a result the city expanded to take in an area called Melton, which had previously been part of a different region.

When Melbourne's boundary was updated, it instantly increased the city's population by tens of thousands of residents.

This also isn't the first time Melbourne has been bigger than Sydney, but in 1905 Sydney took the title after it started to grow much more quickly.

Until now the closest in population the two had ever come was back in 1930, when Sydney had 1.2 million residents compared with 995,000 in Melbourne.

But this boundary change also isn't the only reason Melbourne's population has increased.

The city's growth had been faster than Sydney's for much of the last decade, with more people both from inside and outside the country wanting to move to Melbourne than to Sydney.

Liz Allen, a demographer at the Australian National University, said: "Melbourne has an air to it that diversity seems to be celebrated more," with many people having moved from India to Melbourne, a strong sense of international community has been created.

This over time has made the city appear as a preferable destination for migrants from other countries close to India, and also further afield.

Liz also said Melbourne's cheaper cost of living, particularly when it comes to housing, makes the city a more attractive place to live, especially as both cities have similar opportunities when it comes to education and employment.

Despite the new statistics, people still consider Sydney be bigger than Melbourne, and other ways of measuring population - including looking at the population of the Greater Melbourne or Greater Sydney area - still have Sydney at the front.

This is because the boundary of Greater Sydney takes in a large area of New South Wales where many residents live and commute to Sydney each day.

But it's been predicted that even according to this method, Melbourne is likely to over take Sydney once again as Australia's most populous city by 2031-32.

The population of a given area is often defined as the number of people usually living in that area, measured on 1 January in a given year, according to the most recent census.

A census is normally carried out every 10 years, with a yearly adjustment made in order to calculate the population for that particular year, based on births and death and the migration of people moving in or out of that country.

But how different cities work out their population can vary from country to country.

Defining the borders of a city is also no easy task, with no set international standard to ensure they are all using the same way of calculating.

Richard Greene, Associate Professor of Geography at Northern Illinois University in the US said there are three main different ways to think of a "city".

The different ways of measuring cities has often led to confusion, even for experts and academics - who have even at times found it hard to decide what the list of the 20 biggest cities in the world are, because of many different methods being used.