Its eastern half had three of the five highest yes-voting electorates in the country while its western half had the seven lowest yes-voting electorates.

In all, 12 of Sydney's 29 federal electorates bucked the national trend and voted against same-sex unions.

In the seat of Blaxland, which includes the suburbs of Regents Park, Villawood and parts of Auburn, only 26 per cent voted yes, the lowest share of Australia's 150 electorates. Just 20 kilometres to the east, in the inner-city electorate of Sydney, the yes vote was 83.7 percent, the nation's equal highest.

So why was Sydney - home of the gay and lesbian Mardi Gras - so divided over same-sex marriage?

Demographic differences go some way to explaining the schism.

Religion

The image of Sydney as a fast-paced, expensive global city can be deceiving.

It's also very devout by Australian standards. At last year's census only 24.6 per cent of Sydneysiders ticked the "no religion" box - 5 percentage points lower than the national figure. Hobart's "no religion" share, by comparison, was nearly 40 per cent and Canberra's 36.2 per cent.

As you'd expect, there was a clear correlation between religious beliefs and voting behaviour in the same-sex marriage survey.

Analysis by Jason Ockerby, director of the economic consultancy CEG, found seats with a greater percentage of people identifying as religious were more likely to vote 'no'. This was the case "even after controlling for electorate income," he said.

Religious affiliation is unusually high across western Sydney - only 17 per cent of people in that area ticked the no religion box at the last census compared with 30 per cent nationally.

Some commentary on the high 'no' vote in western Sydney has drawn attention to the region's relatively large Islamic population and the traditional views on marriage held by that community. Muslims make up almost 10 percent of the population of western Sydney compared with 2.6 per cent nationally.

But Matt Grudnoff, an economist at the Australia Institute, said "this is only a small part of the story" because the 'no' vote across Sydney's west averaged around 58 per cent - 20 percentage points higher than the national average.

Western Sydney is also home to sizeable Catholic, Eastern Orthodox and African Christian communities which also seem to have voted for the status quo on marriage. The region also has an above average share of Buddhists and Hindus.

Many seats that voted 'no' had a large population born in non-English speaking countries. But that correlation is complicated by results in the electorates of Sydney and Melbourne which both have very big overseas-born populations but also returned the equal highest 'yes' vote in the nation.

Income

Jason Ockerby found personal income was "an explainer" of voting patterns in the same-sex marriage survey.

"Higher income electorates were significantly more likely to vote 'yes' than lower income electorates," he said.

Many of the Sydney electorates with a 'no' majority have relatively low household incomes compared with the rest of the city.

According to last year's census the median weekly household income in Blaxland, where just one in four voted 'yes', was 15 per cent lower than the national household median and 30 per cent lower than the Sydney-wide figure.

Also, unemployment is relatively high in parts of western Sydney. The three electorates with the highest 'no' vote in Australia - Blaxland, Watson and McMahon - all have above average jobless rates.

Education

In electorates with a big 'yes' majority voters were more likely to be university educated than in seats with a majority 'no' vote.

In the seat of Sydney, which had the equal highest yes vote in Australia, 43.8 per cent of electors had a university degree - double the national average.

Whereas in Blaxland, the electorate with the strongest 'no' vote, the share with a university degree was 17.3 per cent, almost 5 percentage points below the national average.

Marriage

There was another demographic twist in Sydney's same-sex marriage vote.

Macquarie University demographer, Nick Parr, points out that the three electorates in Sydney with the highest vote in favour of same-sex marriage - Grayndler, Wentworth and Sydney - also have the city's lowest share of married couples.

In the seat of Sydney just 24 per cent of people over 15 are in a registered marriage compared with 48 per cent nationally.

The three western Sydney electorates with the highest 'no' vote all had an above average share of married couples.