The "yes" side on same-sex marriage is headed for a resounding victory with seven out of 10 definite voters backing a change to the law, a Fairfax/Ipsos poll has found.
Some 65 per cent of respondents rated themselves "certain" to take part in the voluntary postal survey, and of those 70 per cent said they would vote "yes".
The overall level of support for marriage equality, including those who were not certain they would vote, was also 70 per cent - a figure unchanged since July last year.
The results herald a promising start for same-sex marriage supporters as the Australian Bureau of Statistics starts distributing the first of 16 million survey forms on Tuesday.
But with only two-thirds of Australians inclined to participate, it means same-sex marriage could become law with just 45 per cent of Australians physically casting a "yes" vote.
Respondents were asked how likely they were to participate in the survey, on a scale of one to 10. Only those who rated themselves a "10" were considered certain to vote.
Women were significantly more likely to vote, with 72 per cent saying they would definitely return their survey form, compared to 59 per cent of men.
The expected participation rate of about 65 per cent matches findings from other polls, including one conducted the previous week on behalf of The Equality Campaign.
Same-sex marriage advocates are concerned apathy and complacency could see many supporters forget or ignore their survey forms, especially younger voters.
But the Fairfax-Ipsos poll found expected participation was roughly equal across age groups: 64 per cent of 18 to 24-year-olds rated themselves certain to vote, and 68 per cent of those over 55.
Ipsos pollster Jessica Elgood said she was surprised by that result.
"Generally when you've got non-compulsory voting you see the older cohort turning out to vote," she said.
"What's quite unusual here is we've got equally high likely turnout amongst the younger vote ... which personally I think speaks to the salience of the issue for young people."
Among those who said they were certain to participate, all age groups had majority support for same-sex marriage, including 54 per cent of people over 55.
Support for marriage reform was also multi-partisan, including 52 per cent of Coalition voters, 86 per cent of Labor voters and 94 per cent of Greens.
The poll of 1400 voters was conducted between Wednesday and Saturday last week and had a 2.6 per cent margin of error.
On Monday, the Turnbull government and Labor moved closer to a deal to set ground rules for the unprecedented campaign, to be enshrined in law by the end of the week.
The parties are understood to be debating anti-vilification measures likely modelled on NSW laws that make it unlawful to incite hatred, serious contempt or severe ridicule on the basis of sexuality and transgender status.
Acting Special Minister of State Mathias Cormann said he believed the laws could balance "protecting Australians from vilification, intimidation and threats" with the right to free speech and free expression.
In addition, the bill will import protections from the Electoral Act that usually apply to elections, such as authorisation requirements for campaign material.
Shadow attorney-general Mark Dreyfus was also confident of a deal on anti-vilification provisions, and said the government was negotiating in good faith.
Accusations of vilification have swung both ways. On Monday, Australian Christian Lobby director and prominent "no" campaigner Lyle Shelton, said he was "frankly sick" of being called bigoted by supporters of change.
"People on the 'yes' side routinely get away with vilifying those who believe that marriage should be between a man and a woman," he told Sky News.
"I'm quite frankly sick of this. We are not motivated by hate. We have a difference in public policy about a very important institution in our society, and we should be free to express that without being demonised and vilified."
Also stoking controversy on Monday was former resources minister Matt Canavan, who dismissed a warning from the National Mental Health Commission about the potential consequences of the campaign for LGBTI Australians.
"Can't we just all grow a spine and grow up?" he asked. "The debate hasn't been that bad ... let's stop being delicate little flowers."
Labor's treasury spokesman Chris Bowen said those comments were "pretty unfortunate".
"There are people who are going through great anguish. Mental health in the gay and lesbian community is a very big issue," he told Sky News.
"We keep being told it's a respectful debate, but it's not always a respectful debate."