In a major win for the government, the full bench of the court found the $122 million voluntary mail-in ballot - approved not by Parliament but by executive power - was legal.

The legal win means the government avoids another embarrassing and bitter round of infighting over same-sex marriage, which would have seen moderate Liberals again push for a free vote in Parliament.

Prime Minister Malcolm Turnbull kicked off the two-month campaign by declaring he will vote "yes", and would encourage others to do so, but resisted calls for a bipartisan campaign effort.

In a marked departure from normal Australian democractic processes,, the postal survey will be voluntary, requiring both sides and the Australian Bureau of Statistics to convince voters to participate.

Survey forms will be dispatched by mail from Tuesday, and must be returned by November 7, with a verdict to be announced November 15.

If most Australians vote "yes", Mr Turnbull has promised to facilitate a private member's bill for legalising same-sex marriage, with a view to passing it through Parliament before Christmas.

Finance Minister Mathias Cormann, who has responsibility for the survey and is himself opposed to same-sex marriage, said on Thursday he was "very confident" that could be achieved.

The High Court challenge was brought by two separate marriage equality groups and argued the government had acted unlawfully to fund the survey using a special reserve of the budget, which sets aside money for matters that are "urgent" and "unforseen".

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 relevant law, "on its proper construction, did authorise the Finance Minister to make the determination".

Same-sex marriage advocates said they were 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.

Opposition Leader Bill Shorten called on Mr Turnbull to use his platform to campaign actively, telling him: "When you have a position of leadership in this country, you should lead not hide."

Despite earlier reports, Senator Cormann said he was not aware of any gag order on Cabinet ministers, indicating they would be free to campaign as forcefully as they wished for either side.

Greens leader Richard Di Natale, whose colleague Janet Rice co-sponsored one of the legal challenges, declared: "We're going to work our backsides off and win this bloody thing."

And an emotional Shelley Argent - a same-sex marriage campaigner and a plaintiff in the other challenge - begged the LGBTI community to "please stand strong, because we will win this".

Senator Cormann said it was now "full steam ahead" on the government's election promise of giving the Australian people a say on same-sex marriage, even if via the second-best postal survey rather than a binding plebiscite.

He said Australians were "absolutely able" to conduct the debate respectfully. To that end, the government will next week introduce legislation to give the unorthodox campaign the protections enjoyed by regular elections, including authorisation requirements for campaign material.

Labor indicated it would support that move, but argued the safeguards of the Electoral Act did not go far enough, as they would not protect members of the LGBTI community from vilification.

And Mr Shorten reiterated his promise that Labor would ignore a "no" vote under any circumstances, and make same-sex marriage legal if it won government. "We are absolutely committed to legislating for marriage equality," he said.

Former prime minister Tony Abbott - the original architect of the plebiscite and a prominent voice on the "no" side - derided Mr Shorten's promise as "elitist bullying".

After two years of tortured debate over same-sex marriage within the Coalition, MPs moved to portray the survey as the mechanism to settle the issue "once and for all".

Gay Liberal MP Trent Zimmerman, one of the "rebels" who pushed for a free vote, said the stakes were very high and conceded a "no" vote would kill reform "for many years".

Both sides of the debate fired the starting guns on Thursday afternoon, promising to run respectful campaigns. The Equality Campaign released new TV and radio advertisements, and the president of the Business Council of Australia, Grant King, used the court decision to write to 130 chief executives telling them he would vote "yes" in the survey.

"The Business Council has long supported moves towards a fairer and more inclusive society, and I hope the tone of this debate reflects our values of tolerance and respect," Mr King wrote.

Coalition for Marriage spokesman Lyle Shelton, who ordinarily runs the Australian Christian Lobby, said his campaign would focus on religious freedom and what kids were taught in schools.

"I'm confident in the judgment of the Australian people," he said.