The Turnbull government's postal vote on same-sex marriage has been given the green light, with the High Court rejecting a legal challenge to the controversial survey just six days before ballots are due to be mailed out to millions of households.
The decision will come as a relief to the government, which would have been under renewed pressure to find another way to hold a survey or plebiscite, or settle the matter via a free vote in Parliament, if the postal survey had been derailed.
Instead, about 16 million people on the electoral roll will start receiving ballot papers from Tuesday, which will ask the single question: "Should the law be changed to allow same-sex couples to marry?"
Voters will have until November 7 to return their forms, with a result due on November 15.
The constitutional challenge had been brought by two groups: the first comprising independent MP Andrew Wilkie, Victorian mother-of-three Felicity Marlowe and Shelley Argent from Parents, Family and Friends of Lesbians and Gays; the second involving Australian Marriage Equality and Greens senator Janet Rice.
In a decision lasting only minutes, High Court Chief Justice Susan Kiefel dismissed the plaintiffs' challenges, and ordered them to pay costs. She said the minister had acted within his right in authorising a $122 million advance for the postal survey, telling the court: "Section 10 [of the Appropriation Act], on its proper construction, did authorise the Finance Minister to make the determination."
An emotional Ms Argent choked back tears after the decision, conceding the campaign had been "nasty" and difficult for many.
"We want all the LGBTIQ people in the community to know that we are supporting you, that we have done our best, and that's all that we can do. I just say: please stand strong because we will win this."
Mr Wilkie said: "Of course we all respect the High Court and we respect their decision today. But their decision today doesn't change the fact that this is bad government policy. To be spending the money in the way they are is out of step, at least with the community's expectations with how they should be governing this country."
Ms Marlowe said taking legal action had not been a decision she'd taken lightly - particularly as it risked placing her three children under the spotlight.
"When I first said I would possibly take the government to the High Court to stop this postal plebiscite, they were like: 'Yeah mama, go for it.' We'll be having a very difficult conversation with them tonight, as will many many rainbow families ... We need to win this."
The challengers' cases had focused on the claim that the government had unlawfully financed the postal vote through a special funding pool, which sets aside money specifically for matters that are "urgent" and "unforseen".
Opponents had told the High Court that Finance Minister Mathias Cormann had not met those criteria, in part because Coalition ministers had been publicly discussing "alternative measures" as early as March to deliver on the promise of a public vote.
They also argued the Australian Bureau of Statistics did not have the authority to collect the kind of information requested of it, and the postal vote did not fall under the "ordinary annual services of government".
However, Solicitor-General Stephen Donaghue, QC, argued that, while a compulsory plebiscite had previously been canvassed, a voluntary vote conducted by the ABS had not been decided by cabinet until last month - and therefore had not been foreseen.
He said there was an urgent need for the government to deliver on its policy, and rejected claims that the money should have been appropriated through a vote in Parliament.
Same-sex marriage advocates say they are disappointed the "divisive" postal vote will go ahead, but have vowed to campaign hard for a "yes" vote, buoyed by opinion polls showing the majority of Australians want marriage equality.
The decision was handed down during question time in Parliament.
Opposition Leader Bill Shorten used the occasion to ask Prime Minister Malcolm Turnbull whether he would co-sign a letter to all Australians calling for a "yes" vote.
Mr Turnbull said Mr Shorten should "make his case, and I'll make mine". Mr Turnbull reiterated that he and his wife Lucy "will be voting yes, and I will be encouraging others to vote yes". But "unlike the Leader of the Opposition, I respect every Australian's view", he said.
Mr Turnbull reminded Parliament of Mr Shorten's previous declaration, in 2013, that he was "comfortable" with a plebiscite on same-sex marriage, and said the Labor leader "must be relieved" the poll was going ahead.
In a statement, Attorney-General George Brandis and Mr Cormann said the government would "move swiftly" to implement special legislation to ensure the survey was conducted fairly and properly under provisions that usually apply to elections.
The government had always been confident in its legal advice indicating the postal survey was legal, they said.
Independent MP Jacqui Lambie said she had never supported the postal survey, "not because it's unconstitutional, but because it's dumb".
However, she said she was glad the High Court had thrown out the challenge, so Australians could have their say.
"I will support the majority outcome for Tasmania regardless of my personal beliefs."
Coalition for Marriage spokesman Lyle Shelton, who ordinarily runs the Australian Christian Lobby, promised to run a "respectful campaign" focusing on so-called consequences same-sex marriage, which he said were restrictions on religious freedom and more gender theory in classrooms.
"I'm confident in the judgment of the Australian people," he said.
