Australians look set to be heading to the postbox rather than the ballot box to decide the issue of same-sex marriage.
At a special meeting on Monday, party members voted to back the Coalition's long-standing policy to hold a plebsicite on the issue, to be held on November 25.
A proposal will be resubmitted to parliament this week.
If, as predicted, it again fails, a voluntary postal vote will be held.
If the result of this ballot is "yes", a private member's bill will go to parliament, followed by a free vote.
This will not happen if most respondents vote "no".
Prime Minister Malcolm Turnbull says voters will get the chance to have their opinion heard.
"The important thing is that every Australian gets their say. Every Australian, as Mathias (Cormann) said, on the electoral roll will get a ballot paper and they will be able to fill that in and express their say and their vote..hang on...their vote will be counted."
Mr Turnbull says the Australian Bureau of Statistics would be tasked with running the ballot, at a cost of $122 million.
Papers would be sent out in mid-September, and the result determined by November 15.
The Nick Xenophon Team is among those crossbenchers who have declared they will block attempts to hold a postal plebiscite.
MP Rebekha Sharkie has told Sky news should one go ahead, she will work to ensure everyone gets a say.
"What I'll be campaigning for is for people to vote. I think it's really important that people don't boycott any sort of election or voting opportunity, and a lot of people do that. A lot of people feel disenfranchised and they don't even vote in our general elections. I won't be campaigning for yes or no, but I've made it very clear that I do support marriage equality."
The path to a postal vote is uncertain, with one advocacy group calling it "unconstitutional" and announcing it will launch a High Court challenge.
Co-chair of Australian Marriage Equality, Alex Greenwich, has told the ABC he is concerned about reviving a very public and, at times, unpleasant debate.
"Well we've obviously seen the conduct of the opponents of marriage equality all along and it has indeed been disappointing. Obviously, they seem to have no plans to change their tack. So, I think that the Labor Party is absolutely right in that concern and that concern is shared by many people."
Under pressure, Prime Minister Turnbull insists he is demonstrating his strength as a leader by sticking to his promises.
But Greens leader Richard Di Natale has accused Mr Turnbull of failing to stand up to the hardline conservative elements within his own party.
"If Malcolm Turnbull can't lead the Liberal party, he can't lead the nation. What we're seeing is a Prime Minister who is a huge disappointment as the Prime Minister of this country. And the reason? Because he won't stand up to Tony Abbott and the hard-right in the Liberal party, and if you can't show leadership there, you can't lead the nation."
Also facing a difficult choice, the Opposition will have to decide whether to campaign for the "yes" vote or support a potential legal challenge to a postal plebiscite.
Labor leader Bill Shorten says the issue has snowballed out of control.
He says he believes his view echoes that of most Australians - just get it done.
"And for two good reasons we should have a vote in parliament: one, because it's a good idea; and two, because this nation needs to do some other work, this parliament needs to do some other work. Electricity prices, housing affordability, climate change, low wages, more...dealing with inequality in this country, and yet the Liberal party and the Nationals turn themselves inside-out over marriage equality."
