Prime Minister Malcolm Turnbull told reporters on Tuesday he would "give all Australians a say" on whether same-sex marriage should be legal.
"I'll be voting yes, as will (my wife), I'm very open about that but the Australian people are never wrong when they vote, whether it's for governments or on matters like this, their vote will be respected," Turnbull said.
The governing Liberal National coalition failed Wednesday morning to pass legislation to hold a compulsory national vote, or plebsicite, on same-sex marriage. It had already attempted once in October 2016
Turnbull said Tuesday if Parliament rejected the plebiscite a voluntary national postal vote would be held instead without parliamentary approval.
The Australian leader said the postal vote would be completed before the final sitting week of Australia's Parliament in November.
It has been widely condemned by same-sex marriage advocates, who say the vote is unnecessary and expensive, while a polarizing campaign would be traumatic for Australia's LGBT community.
The opposition Labor Party has repeatedly called for a simple vote in Parliament to approve same-sex marriage, which consecutive polls show most Australians support.
"Our family and friends deserve better. Marriage equality is about real people, our friends and family, teammates and work colleagues who just want the same dignity as everyone else," Anna Brown, co-chair of the Equality Campaign, said in a statement
Legal action possible against vote
The latest attempt to resurrect the debate around same-sex marriage was triggered after a group of rebel politicians inside the government
began a push to ditch the plebiscite as official government policy.
A meeting of politicians in the Liberal Party was held on Monday, where it was decided there would instead be a renewed effort to formulate a national vote, followed by a postal vote, if it was unsuccessful.
The bill to fund the plebiscite was always unlikely to pass the Australian Senate on its second attempt due to continued opposition from a range of parties.
"Labor, Green (and) some of the independent senators, as well, (gave) no signs they'd changed their mind on it," John Warhurst, emeritus professor at Australian National University's School of Politics and International Relations, told CNN Monday.
Despite a postal vote now appearing all but certain, advocacy groups are promising a legal challenge, local media reported.
"The issue is can the government pay for a postal plebiscite in a constitutionally valid way, and I think there's serious questions about whether it can do that," Paul Kildea, director of the Referendums Project at the Gilbert + Tobin Centre of Public Law, told CNN.
Kildea said without parliamentary legislation to approve the expenditure, there could be questions around whether they had the "statutory authority" to carry it out.
Finance Minister Mathias Cormann told reporters the cost of a postal vote would be $122 million. The result of the postal vote would not be binding on members of the Australian Parliament.
Turnbull told reporters on Tuesday they had legal advice that the postal vote would stand up to any challenge in Australia's High Court.
Postal vote would begin September 12
If the government chooses to hold a national postal vote, Cormann said he hoped the ballots would start being delivered on September 12, for a decision by November 12.
Australia is one of only a few English-speaking countries where same-sex marriage is not legal, lagging behind the United Kingdom, Canada, South Africa and the United States.
The plebiscite was first raised as a possibility at a meeting of the Liberal National Coalition in August 2015, where then-Prime Minister Tony Abbott promised the national vote after the 2016 election.
"The Coalition supports the traditional definition of marriage between a man and a woman, but we won't seek to bind people beyond this term of Parliament and in the next term ... it will be the people's decision," Abbott said at the time
Commentators and same-sex marriage advocates at the time saw it as a move by conservative Abbott to postpone the issue as long as possible.