While there has been growing support for marriage equality, with 70% of those surveyed in a new Fairfax Media poll on Tuesday backing the "yes" campaign, Australia has not yet legalised such unions despite more than a decade of political wrangling.
The conservative government chose an unusual approach - a voluntary and non-binding postal vote - after an election promise of a national plebiscite was knocked back twice by the upper house Senate.
"I encourage everyone to fill in the survey and return it. I'll be voting 'yes' as will (my wife) Lucy," Prime Minister Malcolm Turnbull told commercial radio this week.
Turnbull, a moderate, is opposed by some members of his conservative ruling Liberal-National coalition on legalising gay marriage, with the postal vote seen as a compromise.
The start of the ballot process followed rallies on the weekend for and against changing marriage laws, with thousands of people dressed in rainbow colours packing central Sydney on Sunday to back the "yes" vote.
Hundreds of "no" campaigners marched on Saturday, arguing that changes would infringe on religious freedom and children's rights.
Up to 15 million Australians will be asked: "Should the law be changed to allow same-sex couples to marry?" on the ballot paper and given the option of marking "yes" or "no" boxes.
But "yes" campaigners have warned the method of collecting votes, via the postal system, could be less effective at engaging younger tech-savvy Australians, who are seen as more supportive of changing the laws.
National Party MPs have also voiced concern about Australia Post's abilities to deliver the survey to rural and regional areas across the vast country.
The postal vote will close on November 7, with the result released on November 15.
If most Australians vote "yes", the government will move for a parliamentary vote, but will not do so if there is a "no" outcome.
The survey has shed light on how rare national non-binding plebiscites are in Australia. The votes are different in the country to referendums, which affect the constitution.
Just three have been held - two on conscription in 1916 and 1917, and one on which song should be the national anthem in 1977.
But all occurred under the Electoral Act, unlike the postal survey which is being carried out by the Australian Bureau of Statistics.
The bureau was given the task by Canberra to avoid the need for parliamentary approval, but that meant the marriage poll did not attract advertising protections against malicious campaign material in force during elections - a key concern for activists fearing it could unleash a barrage of homophobia.
Politicians this week were scrambling to pass similar election-type safeguards restricting material that might be misleading and deceptive.