The largest gay rights rally in Australian history was held in the eastern city of Sydney on Sunday, demanding legalization of same-sex marriage, as political leaders urged voters to back the issue in a postal survey that starts on Tuesday.

Equal Marriage Rights Australia said 40,000 people marched in support of same-sex marriage in Sydney, waving rainbow flags and signs reading "yes." Similar rallies were held in other cities.

The march comes as all 16 million registered voters are to be sent a postal vote starting on Tuesday asking the question: "Should the law be changed to allow same-sex couples to marry?"

Voters will check "yes" or "no," and if the former predominates, parliament will debate legalizing same-sex marriage before Christmas. The results of the postal vote will be announced on November 15.  

Turnbull campaigns for 'yes'

Before the rally, Prime Minister Malcolm Turnbull threw his support behind the "yes" vote, telling members of the Liberal and National parties to "be honest with each other" that gay marriage was not a threat to traditional marriages.

"I am utterly unpersuaded by the proposition that my marriage to Lucy, 38 years long next March, or indeed any marriage, is undermined by two gay men or two gay women setting up house down the road, whether it is called a marriage or not," ABC News reported Turnbull as saying.

The prime minister also reminded supporters that same-sex marriage has already been approved in 23 countries.

"In any one of those nations, has the sky fallen in? Has life as we know it come to a halt? Has traditional marriage as we know it been undermined? The answer is no," he said.

Turnbull has said before that he supports same-sex marriage, but had not signaled he would publically campaign for a "yes" vote.

Opposition Labor Party leader Bill Shorten, speaking to the same-sex marriage rally at Sydney's Town Hall, welcomed Turnbull's support and said it was "time to make marriage equality a reality."

Public backing

The results of the postal vote are not binding on members of parliament, who can still vote against the measure when it comes up for debate.

According to a recent poll, nearly 58 percent of those surveyed said they would back same-sex marriage, while about 31 percent were against and 10 percent still undecided. The poll showed that 65 percent of voters are likely to participate in the postal vote. 

The postal vote has prompted criticism from some who argue it is a waste of money and parliament should just vote on the issue. A majority of lawmakers support same-sex marriage.

cw/tj (Reuters, dpa)