Same-sex couples could be left waiting until the 2020s to get married if Labor blocks the plebiscite proposed for February next year, Attorney-General George Brandis says.
In a blunt warning to the federal opposition, which is all but certain to oppose the national vote, Senator Brandis suggested the political will to change the law could dissipate if the plebiscite is not held in this term of parliament and same-sex couples could be forced to wait well beyond the next election, due in 2019.
That could mean the law change was pushed back until the next decade.
The warning came in an interview with Fairfax Media ahead of a crunch meeting on the plebiscite in Brisbane on Monday, at which Senator Brandis, Special Minster of State Scott Ryan, Labor legal affairs spokesman Mark Dreyfus and Labor shadow minister for equality Terri Butler will discuss the plebiscite.
Ahead of the meeting, Senator Brandis said the government – which recently struck a deal with Labor to pass the omnibus savings bill – would look to negotiate in good faith and warned the hopes of gay couples could otherwise be dashed.
"The government is prepared to negotiate with the Labor Party in good faith because we are committed to resolving this matter by a plebiscite. That was endorsed by the Australian people at the last election," he said.
"And if, like me, you favour reform to the Marriage Act to allow same-sex couples to marry then this is now the only feasible path to that outcome for many years to come."
The warning has been echoed by Australians for Equality director Tiernan Brady, who said blocking the plebiscite could see the legalisation of same-sex marriage delayed "indefinitely" and urged the major parties to hammer out a compromise.
It also comes as new research from the University of Melbourne suggested just one electorate in Australia, the rural Queensland seat of Maranoa, had a majority of voters opposed to same-sex marriage while just a handful more – Groom and Flynn in regional Queensland, Hinkler further north and Deputy Prime Minister Barnaby Joyce's base in New England – might oppose the change in a plebiscite scenario.
The study found opposition to changing the Marriage Act ranged from 40 to just over 50 per cent in a handful of rural Queensland and northern NSW seats to less than 10 per cent in inner-city electorates in Sydney and Melbourne.
The study was based on responses to the ABC's Vote Compass survey in 2013 and weighted to reflect the demographics in each electorate. Overall, just a third of voters were opposed to a change.
Senator Brandis' comments are designed to serve as a warning to the ALP and to same-sex marriage advocates who oppose a plebiscite and believe Coalition MPs will be allowed a free vote in the parliament if the plebiscite push fails.
Such a move would potentially split the Coalition and endanger Malcolm Turnbull's leadership and is all but certain to not occur.
He also said he was open to further discussions with cross bench senators such as the Nick Xenophon Team and independent Derryn Hinch, who currently oppose the plebiscite, if Labor would not compromise.
The federal opposition has not formally said it will block the plebiscite but is expected to do so when the caucus meets later this month.
Ahead of the meeting Ms Butler said the opposition would listen to Senator Brandis' pitch but re-stated a raft of concerns the party had.
Those concerns included "the implicit exclusion of LGBTI people, the harmful and divisive nature of a plebiscite, the [estimated $170-$200 million] cost of the plebiscite, the public funding of the 'yes' and 'no' cases, the fact that it is a radical departure from how we do democracy in Australia, and the non-binding nature of the vote on Liberals."
Mr Brady said gay couples now faced the very real scenario that "there will be no clear pathway to marriage equality, even though the leaders of the three major parties are in favour of it, a majority of parliamentarians are in favour and the majority of Australians are in favour."
"It would be a real tragedy to have three marriage equality bills and no legalisation of marriage equality," he said. "We want this to be cross party and it needs to pass by whatever pathway."
One of the researchers, Monash University political scientist Shaun Ratcliff, said that while the study was not a direct poll of voters it showed the vast majority of electorates wanted change.
"Based on our data and other polling, it appears a plebiscite would pass easily," he said.
The Melbourne University study found a significant divide in opinion between rural and urban areas. Every rural and regional electorate in the country has below-average backing for change, while support is above average in inner-city seats of every capital city.
It also found the stronger the support for legalising same-sex marriage in an electorate, the more likely an MP was to publicly support the change. There was a significant shift between 2012 and 2015, with MPs from both major parties changing their position, but politicians were still lagging behind the public.
Ten electorates most opposed to same-sex marriage
Maranoa*, QLD (LNP, David Littleproud)
Groom, QLD (LNP, John McVeigh)
Flynn, QLD (LNP, Ken O'Dowd)
Hinkler, QLD (LNP, Keith Pitt)
New England, NSW (Nationals, Barnaby Joyce)
Kennedy, QLD (Katter's Australian Party, Bob Katter)
Grey, SA (Liberal, Rowan Ramsey)
Barker, SA (Liberal, Tony Pasin)
Parkes, NSW (Nationals, Mark Coulton)
Braddon, TAS (Labor, Justine Keay)
*The only seat where a majority was opposed.
Ten electorates most supportive of same-sex marriage
Sydney, NSW (Labor, Tanya Plibersek)
Melbourne, VIC (Greens, Adam Bandt)
Grayndler, NSW (Labor, Anthony Albanese)
Wentworth, NSW (Liberal, Malcolm Turnbull)
Melbourne Ports, VIC (Labor, Michael Danby)
Wills, VIC (Labor, Peter Khalil)
Gellibrand, VIC (Labor, Tim Watts)
Batman, VIC (Labor, David Feeney)
Higgins, VIC (Liberal, Kelly O'Dwyer)
Brisbane, QLD (Liberal, Trevor Evans)
