The enrollment deadline was confirmed in treasurer Scott Morrison's directions to the Australian Bureau of Statistics.
Same-sex marriage ballot papers will start arriving in the mail from September 12, with a return deadline of November 7.
Australians have two weeks to register to vote or update their details, which they can do by visiting the AEC website.
Around 14 per cent of young people aged between 18 and 24 are not currently enrolled, according to the latest data from the Australian Electoral Commission.
Nationally, the percentage of Australians of all ages who are not enrolled is around 5 percent.
A final result from the poll is expected by November 15.
In the event of a 'yes' vote, a private member's bill to legalise same-sex marriage will be introduced to the house in the final sitting fortnight of 2017.
But if the people vote "no", Prime Minister Malcolm Turnbull said no bill would proceed.
Australians abroad will be mailed ballot papers
The government has assured Australians living overseas they will get their say.
"All Australians who are enrolled on the Commonwealth Electoral Roll will get the opportunity to express their view through the postal plebiscite process," Acting Special Minister of State Mathias Cormann told SBS World News earlier this week.
"Where Australians overseas, who are on the Electoral Roll, have registered as an overseas voter and provided their overseas address, then the request for response will be sent directly to them."
Australia House in London took over 15,000 votes for the 2013 federal election, according to the Australian Electoral Commission.
High Court challenge looms
An independent federal MP and a group of same-sex marriage advocates will lodge a legal challenge to the postal vote with the High Court this week.
The action will be brought by Tasmanian independent MP Andrew Wilkie and same-sex marriage advocates Shelley Argent and Felicity Marlowe.
The Turnbull Government has instructed the Australian Bureau of Statistics (ABS) to run the poll, at a cost of $122 million.
Running it through ABS will allow the government to hold the vote without passing legislation, after the full, in-person plebiscite was twice blocked in the Senate.
But long-term same-sex rights advocate Rodney Croome said legal advice provided by barrister Ron Merkel QC found there were constitutional problems with the ABS running the poll.
"Mr Merkel feels that the idea of a postal vote running through the ABS may exceed the ABS's authority, particularly when we consider whether a postal vote on marriage equality is statistic gathering or not," Mr Croome said.
The original plebiscite, which would have cost an estimated $170 million, was defeated a second time on Wednesday when Labor, the Greens and the Nick Xenophon Team had the numbers to tie a vote at 31-all.
