The survey does not have the power to change the law but could lead to a vote in parliament.
Australian political leaders, including Prime Minister Malcolm Turnbull, have said they support the Yes campaign.
People will have until 7 November to cast their vote by mail and results of the voluntary poll are expected on 15 November.
On Sunday ahead of the ballots being mailed out, thousands of Australians rallied in support of the campaign and dressed in rainbow colours.
The No campaign said it remained confident it had strong support.
Mr Turnbull said the country's Marriage Act could be amended this year if the vote showed support for change.
"We encourage every Australian to vote in this survey," Mr Turnbull said last week.
The postal survey has drawn controversy over its $A122m cost, fears that it would prompt hate-filled campaigns, and the fact that voting was voluntary.
It has also been criticised by same-sex marriage advocates as an unnecessary delay in resolving the debate, which they say could be settled immediately in parliament.
The High Court of Australia last week dismissed two separate objections to the validity of the poll.
Both sides have been fiercely campaigning. One No advert was criticised for linking the debate to school gender education, while Yes campaigners have been accused of trying to silence their opponents.
A poll released on Tuesday suggested that 70 percent of people who rated themselves "certain" to vote were in favour of changing the law.
Photo caption: Australian street artist Scottie Marsh paints a mural of former Australian prime minister Tony Abbott wedding himself in Sydney after Mr Abbott urged Australians to "protect the family" with their vote in the same-sex marriage postal survey.