Both federal leaders have thrown their support behind the "Yes" campaign, days ahead of ballots for the postal survey on same-sex marriage being delivered to more than 16 million Australian voters.

Despite vowing not to campaign, the prime minister launched 'New South Wales Liberals and Nationals for Yes' in Sydney alongside NSW premier Gladys Berejiklian and former NSW Liberal leaders, Kerry Chikarovski, John Brogden, Barry O'Farrell and Nick Greiner on Sunday.

Turnbull quoted former UK prime minister David Cameron saying he doesn't support gay marriage despite being a Conservative, he supports gay marriage because he is a Conservative.

He told the launch there was no evidence that gay couples pose any threat to marriage.

"I have to say I am utterly unpersuaded by the proposition that my marriage to Lucy, 38-years long next March, or indeed any marriage, is undermined by two gay men or two gay women setting up house down the road whether it is called a marriage or not," Turnbull said.

"Let's be honest with each other, let's be really, really honest with each other, the threat to marriage is not gay couples it is a lack of love and commitment." Read more via Buzzfeed