While there has been growing support for marriage equality, with 70 percent of those surveyed in a Fairfax Media poll on Tuesday backing the "yes" campaign, Australia has yet to legalise 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 twice rejected by parliament's upper house, the Senate.
If most Australians vote "yes" to same-sex marriage the government will move to hold a parliamentary free vote on changing the marriage laws. It will not do so if there is a "no" outcome.
"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 the issue and the postal vote is seen as a compromise.
The start of the ballot process followed weekend rallies 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 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 said this 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 ballots to rural areas across the vast country.
- Anti-hate laws -
National non-binding plebiscites -- different to referendums, which affect the constitution -- are rare in Australia.
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 were held 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 to avoid the need for parliamentary approval, but that meant the marriage poll did not attract advertising protections against malicious campaign material which are in force during elections. This is a key concern for activists, who fear the vote could unleash a barrage of homophobia.
Politicians this week were scrambling to pass election-type safeguards restricting material that might be misleading and deceptive.
Among the proposed penalties are fines of up to Aus$12,600 (US$10,100), The Sydney Morning Herald reported.
"It will be unlawful to vilify, intimidate or threaten to harm a person either because of views they hold on the survey or in relation to their religious conviction, sexual orientation, gender identity or intersex status," a government spokesman told the newspaper.
The laws -- which are expected to attract bipartisan support from the government and the main Labor opposition party -- will apply to advertising, leaflets or behaviour during the campaign period, the Herald added.
(This story has not been edited by NDTV staff and is auto-generated from a syndicated feed.)