The full bench of the High Court has given the Australian government's postal survey on same-sex marriage a green light to go ahead, dismissing two legal challenges against the controversial ballot.
The decision means survey forms will be posted out from September 12, with a return deadline of November 7. The result of the national survey will be announced on November 15.
The court handed its decision down on Thursday afternoon in Melbourne.
The seven judges unanimously dismissed the case brought by independent MP Andrew Wilkie, lesbian mother of three Felicity Marlowe, and Pflag's Shelley Argent, ordering that the plaintiffs should pay costs.
The case brought by Australian Marriage Equality and Greens senator Janet Rice was also unanimously dismissed. The court ruled that the government had legally used the Advance to the Finance Minister to spend $122 million on the survey and ordered the plaintiffs to pay costs.
Over the two-day hearing, lawyers argued about whether or not the government can legally spend $122 million on the survey with passing legislation through the parliament, whether the Australian Bureau of Statistics has the legal authority to collect opinions about same-sex marriage, and whether the plaintiffs had the right to bring forward the case.
On Tuesday, finance minister Mathias Cormann announced that the printing of survey forms had begun. The ABS told a senate committee on Thursday morning that $14.1 million had already been spent on the mass survey.
The survey forms will ask Australians to answer the question: "Should the law be changed to allow same-sex couples to marry?" If a "yes" vote is returned, the government will allow a conscience vote in parliament on a bill for same-sex marriage. If a "no" vote is returned, the government will continue to block any legislation for same-sex marriage.
Human Rights Law Centre director of legal advocacy, Anna Brown, called on Australians to vote "yes" outside the Commonwealth Law Courts in Melbourne. "It's time to move forward as a nation, to vote 'yes', and to deliver marriage equality for gay and lesbian people across Australia, their friends and their families," she said.
Argent delivered a message to the LGBTI community: "I want all the LGBT people in the community to know that we are supporting you... Please stand strong, because we will win this."
Independent MP Andrew Wilkie criticised the postal survey, saying he respects the decision of the court but "their decision today doesn't change the fact that this is bad government policy".
The "yes" campaign reacted immediately to the decision with a video ad:
Meanwhile, leading "no" group the Coalition for Marriage welcomed the decision of the High Court.
"It is extraordinary that those pushing to redefine marriage went as far as taking the government to court to stop the Australian people having a say," said Marriage Alliance CEO Damian Wyld.
Catholic Archbishop of Sydney Anthony Fisher said the change to the Marriage Act would affect everybody.
"It is only fair that all Australians are allowed to make their voice heard," he said.
Moments after the judgment was handed down, opposition leader Bill Shorten asked prime minister Malcolm Turnbull to "actively" campaign for a "yes" vote and write a joint letter with Shorten to all Australians calling for them to vote "yes".
Turnbull said he would vote "yes", but would not say whether he would agree to the offer.
"We encourage every Australian to vote in this survey, to have their say, and as I have said in this house and in many other places, Lucy and I will be voting 'yes' and I will be encouraging others to vote 'yes', but, Mr Speaker, above all, I encourage every Australian to have their say because unlike the leader of the opposition I respect every Australian's view on this matter," he said.
"And I thoroughly reject the way in which he has sought to vilify and demonise people who have a different view to him."
Attorney-general George Brandis told the Senate that there was "now no legal impediment to a postal survey proceeding".
"The outcome of the High Court proceedings is what the government expected and is consistent with the advice provided to the government by the Commonwealth solicitor-general, Dr Stephen Donaghue QC," he said.
"On behalf of the government, might I take this opportunity to thank and congratulate Dr Donaghue and his team for their skilful advocacy and sound advice."