Prime Minister Malcolm Turnbull says same-sex marriage could be legalised by the end of the year if the majority of Australians vote yes in the postal survey.
Malcolm Turnbull has joined past and present NSW premiers in supporting the "yes" vote in the upcoming same-sex marriage survey.
The prime minister told NSW Liberals and Nationals for the Yes campaign launch in Sydney he'll be voting yes and same-sex marriage could be legalised by the end of the year.
If the majority of Australians also vote yes, a private member's bill will be presented which will "sail through the parliament", Mr Turnbull said on Sunday.
"Fundamentally this is a question of fairness," he said.
He is "utterly unpersuaded" by the idea that his 38-year-long marriage to wife Lucy is undermined by gay couples.
"The threat to marriage is not gay couples, it is a lack of loving commitment," he said.
"Whether it is found in the form of neglect, indifference, cruelty or adultery.
"If the threat to marriage today is lack of commitment then surely other couples making and maintaining a commitment sets a good rather than a bad example."
Mr Turnbull noted other countries where same-sex marriage has been legalised.
"In any one of those nations has the sky fallen in, has life as we know it ground to a halt, has traditional marriage been undermined and the answer is plainly no," he said.
Premier Gladys Berejiklian said alongside former NSW premiers Nick Greiner and Barry O'Farrell and previous party leaders Kerry Chikarovski, Peter Debnam and John Brogden at the event.
Mr O'Farrell said love is a pursuit that's far more important than politics.
"As a supporter of marriage I want marriage to be on offer to every loving couple in this country," he said.
"In answer to Tina Turner's question, for me this issue has got everything to do about love."
Tony Abbott's sister, Councillor Christine Forster, reflected on a traditional wedding she attended on Saturday where the priest spoke at length about marriage binding two people together.
"In all of that sermon he did not say one thing to that congregation that said this cannot be between two men or two women," she said at the launch.
"It is just about two people.
"That's what this whole discussion is about."
Ms Forster spoke of her decision to marry fiancee Virginia at the British Consulate at Circular Quay in February 2018.
She was hopeful same-sex marriage would be legalised by the end of the year, meaning the pair can instead be married under Australian law.
"That is what we want to do and that is what so many other Australians want to do."