The non-binding and voluntary ballot of nearly sixteen million people will run until November.
An Ipsos/Fair fax judgment poll has propose that 65% of qualified voters intend to take part in the survey – and of those, 70% said they would support gay marriage.
If the postal view finds that most Australians are in favour, Parliament will vote on legislation to raise the prohibition on gay marriage.
However, many lawmakers have warned that they will vote against such a law – regardless of public opinion.
On Sunday, 20,000 people took to the streets of Sydney in support of a Yes vote, occasionally meeting opposition display bearing signs in favour of “traditional” and “natural” marriage.
Opposition leader Bill Shorten, who supports same-sex marriage, told the Sydney rally that the change was about Australia’s “personality as a nation”.
He said: “Do we believe in equal care for all, equal rights for all? We can win this thing!”
The campaign against changing the law has focused on issues beyond marriage itself, including gender education in schools and the “right” of children to have a mother and a father.
A video created by the No campaign shows a mother expressing concern that her son was allowed to wear a dress to school, and claims “in countries with gay marriage, parents have lost their right to choose”.
The postal vote, which offers a yes or no response to the question of whether the law should be changed to allow same-sex couples to marry, was called by Malcolm Turnbull’s conservative government.
The ballot’s unusual format was decided after the election promise of a national vote was rejected twice by parliament’s upper house.
Mr Turnbull has urged Australians to vote Yes, telling reporters that “the right to marry is a great ideal as much as any other principle”.
Two past conservative prime ministers, Tony Abbott and John Howard, have publicly opposed the change.
The vote will close on 7 November, and the result will be released on 15 November.
