The postal plebiscite will be completely voluntary and run by the Australian Bureau of Statistics.
"Parties can have their platforms but when the Parliament of the nation speaks, one normally - unless you're totally disrespecting the Parliament, and unless you just don't care what people say and do and the votes that are taken in Parliament - parties will adjust what they've said before".
Prime minister Malcolm Turnbull has raised the stakes in the debate over same-sex marriage, saying that if the government's plan for a non-compulsory postal vote is struck down by legal action, there won't be a vote on the issue in parliament.
Supporters have called for a parliamentary vote as the best way to legalise gay marriage, with some arguing a public ballot should be boycotted as it would open the door to hate speech against the LGBTI community.
Australia's prime minister has promised a national vote on same-sex marriage before the end of the year as a push towards legalising gay unions threatens to bring down his government.
A final result from the postal poll is expected by November 15.
Despite a postal vote now appearing all but certain, advocacy groups are promising a legal challenge, local media reported.
Australia's first plebiscite in 1916 - on military conscription - cost taxpayers £81,113, or roughly $162 million in current dollars.
Earlier this week, lawmakers in the Liberal Party - the (actually conservative) ruling party in Australia's Parliament - had attempted to pass a mandatory plebiscite, which would have required Australians to vote on the issue at the polls this November.
"If the Liberal Party can not provide a pathway to preserving marriage, freedom and gender norms, millions of Australians who support the Marriage Act will have no choice but to look for candidates with the resolve to preserve it", Mr Shelton told The Australian.
The benefit of the national vote was that all Australians would be part of the decision-making process and the losing side was more likely to accept the outcome.
"[What] two people in love who want to get married has to do with freedom of speech I have no idea".
"Politicians have no special insight into the workings of the human heart and this is an issue so intimate, so personal, that it is nearly uniquely the sort of issue that we want every Australian to have their say", Senator Brandis told ABC TV. Bernardi has said he would vote against gay marriage regardless of what the plebiscite found.
"I'll be talking with my electorate, with the people in my constituency and I'll be encouraging them to lodge a postal vote", she told reporters. "The only reason it has not been held is because of [Labor leader] Bill Shorten's opposition".
"Obviously I will be voting no", Abbott said.
"Why is it that the Liberal Party spend all of their agility and innovation on working out ways to delay marriage equality, I wish they'd put the same effort into electricity prices".