The High Court has cleared the way for forms for the voluntary postal survey on same-sex marriage to go out next week as planned.
The "underdogs" who lost a fight in Australia's highest court to stop the government's postal survey on same-sex marriage are now focused on winning the bigger battle.
Forms for the voluntary survey will go out from Tuesday as planned after the bid to have the High Court stop the $122 million process failed.
The disappointed same-sex marriage advocates behind the case are now determined to campaign hard to get a strong "yes" result.
Australian Marriage Equality co-chair and plaintiff Alex Greenwich said Thursday's decision will mobilise the many supporters of same-sex marriage around Australia.
"This is going to be a tough campaign but we are in it to win it," Mr Greenwich told reporters.
"But certainly the task ahead is daunting and clearly, having had this process imposed upon us in this circumstance, we are clearly the underdog."
The two court challenges argued the government did not have the power to conduct the postal survey because it funded it without getting parliamentary approval and also questioned the role of the Australian Bureau of Statistics in it.
The government found the $122 million by using laws to make an advance payment to the finance minister in circumstances where there is an urgent need for spending and the situation was unforeseen.
The challengers argued the spending was neither urgent nor unforeseen, two key requirements for advancing money from the pool of funds that can be used without parliamentary approval.
The full bench of the High Court unanimously dismissed the first challenge by a group of advocates led by independent MP Andrew Wilkie.
The seven judges answered the questions of law raised by the second group led by Australian Marriage Equality, but unanimously declared the finance minister's determination was valid and he did have the authorisation to make it.
A teary Shelley Argent, who was part of the Wilkie case, said the advocates did their best.
"We had to try to make this work because we know that with the postal survey that it will get very nasty, and probably on both sides, and we don't need that," the Parents and Friends of Lesbians and Gays national spokesperson said.
Ms Argent told the LGBTIQ community: "Please stand strong because we will win this."
The Coalition for Marriage group behind the controversial anti-same-sex marriage commercials welcomed the court decision, saying it would allow the voice of the Australian people to be heard.
Spokesman Lyle Shelton, the managing director of the Australian Christian Lobby, said the coalition would continue to campaign against same-sex marriage until the survey was completed and urged prospective no-voters to donate to their lesser-resourced cause.
Prime Minister Malcolm Turnbull said every Australian will now have their say on the issue.
"That is as it should be. We encourage every Australian to vote in this survey, to have their say," he said.
Mr Wilkie said the High Court decision did not change the fact that the voluntary postal survey was a bad government policy.
The voluntary survey was the government's plan B after the Senate blocked the compulsory plebiscite promised by the coalition at the 2016 election.