Prime Minister Malcolm Turnbull declared his support for the 'Yes' vote in the upcoming same-sex marriage postal survey ahead of a major rally in Sydney on the issue.
The postal survey, which is being run by the Australian Bureau of Statistics, is expected to be mailed to households across the country from early this week.
The survey is going ahead after the full bench of the High Court dismissed a case against the coalition-developed plan and ruled the postal vote could proceed.
Speaking on Sunday, Turnbull said he hoped the 'Yes' vote would succeed.
"My position on this issue has been clear for many years. I am a supporter of same-sex marriage and I'll be voting 'Yes'," he told a conservative 'Yes' campaign event.
"The reality is that the vast majority of Australians respect each other and understand this is an important issue."
Turnbull said if the 'Yes' vote was successful the federal government would ensure a private member's bill was presented to parliament in favour of gay marriage.
He said he was confident such a bill would "sail through parliament".
"I'm voting 'Yes' because fundamentally this is a question of fairness," Turnbull added.
The Prime Minister's comments came ahead of a major rally in Sydney in support of marriage equality.
The march, which coincides with the official launch of the 'Yes' campaign, kicked off outside Town Hall, in Sydney's CBD, at 1pm on Sunday.
Labor and the Greens have consistently backed a free parliamentary vote to make gay marriage a reality, but the coalition has opposed this approach in favour of the postal survey.
Thousands of supporters are attending in addition to opposition leader Bill Shorten, NSW MP Alex Greenwich and 'Yes' Campaign spokesperson Kerryn Phelps.
Shorten addressed the crowd to push his support for marriage equality.
"What this world needs, and what this country needs, is we need help to maintain families, we need help to raise children, and that is why we need marriage equality," Shorten said.
"And when people say this survey is about identity, my word it's about identity.
"We can win this thing. We can climb this mountain. We can participate in this survey. We can vote yes."
Many advocates of same-sex marriage are urging Australians to take part in the $122 million voluntary ballot after a long-running stalemate on the issue.
GetUp activist Sally Rugg told the Sydney rally the 'Yes' campaign would make history in Australia.
"I'm so angry that this government is putting my existence to a postal vote," Rugg said to cheers from the crowd of over 1,000 gay marriage supporters.
"This is not where we want to be."
She said the "conservative elites" were wrong to think gay marriage supporters would take the postal vote "lying down".
"They haven't been paying attention to who we are," Rugg added.
Greens Senator Lee Rhiannon said it was "extraordinary" to be part of the event.
"It's a real reminder of the fact that we get this done, get it over with, and get marriage equality," Rhiannon told HuffPost Australia.
"We know the majority of Australians want it but people have become very cynical about politicians and political parties.
"We really need to follow through with the final stage and give support to LGBTI people."
While Sydneysider Liam McMahon said, as a young gay man, it was important to attend the landmark rally.
The 23-year-old said he wanted the law changed so he would have the option of marriage in the future.
"It's something I'd like to be able to do," McMahon told Huffpost Australia.
He described the postal vote as the best way for marriage equality advocates "to be taken seriously".
"It's the best chance we got, and it's the only chance we've got right now."
Warren, 26, from the Queensland country town of Wawrick, also urged Australians to vote 'Yes'.
"It should be fair, it should be fair for everyone, there's nothing wrong with it at all, it's equality," he said.
"It feels amazing to be here, everyone needs to be equal."