In a bid to circumvent Parliament, the vote will be carried out by the Australian Bureau of Statistics, which means it will not be regulated under Commonwealth Electoral Act rules.
That is - nearly every Australian.
Currently Australia defines marriage as being between one man and one woman, but campaigners have been arguing for a redefinition.
If you're traveling overseas between September 12 and November 7, it'll be pretty hard to vote.
While other countries have decided on same-sex marriage in the courts or in parliament, lawmakers in Australia have spent months bickering about mechanisms.
'It's unscientific, it doesn't conform to the rudimentary requirements of polling, ' he said.
Within just a couple of hours of being uploaded, the video had been viewed nearly 300,000 times, shared nearly 18,000 times, and attracted around 2000 comments.
"It's hurtful as someone who this means a lot to".
If you don't have a permanent roof over your head, you probably don't have a letter box either. A Australians aged 18 and over who are registered to vote.
Citizens who have a registered address overseas will also receive ballots.
'I think this gives us some hope the facts that we've raised and the issues we've expressed about why this postal plebiscite is going to be so detrimental and distressing are going to be considered and thought about, ' she said in Melbourne.
"I have grown and learnt not only to tolerate, but to accept all people regardless of their sexual orientation or any other trait or feature which makes each of us different and unique".
But despite the father-of-two's anger at the matter being put to a postal vote, he implored his fans not to boycott the vote in a typed message at the end of the video.
If you are not on the electoral roll you need to enrol here.
'I wasn't sure at first if it was mandatory, ' he said.
Mr Kirby said the postal ballot was "just a complete political improvisation and it's completely unacceptable and it should stop". "There is a solution, but it does cost money".
Ballots will be distributed to Australians who meet the registration deadline, August 24.
The acting special minister of state, Mathias Cormann, told Fairfax Media the government wanted the postal plebiscite process to be fair.
Megan has been living and working in regional Indonesia for four years. If they get away with it now, we will indeed be on a slippery slope leading to dictatorship.
"The choice is whether we allow these statements to go unresponded to".