Voting for the same-sex marriage postal survey closed on Tuesday afternoon, with 12.6 million people taking part.
The figure accounts for 78.5 per cent of Australians eligible to vote.
Voting closed at 4.30pm AEDT on Tuesday.
"We know many Australians like to work to a deadline - well that deadline has well and truly arrived," Alex Greenwich, a spokesman for the Equality Campaign said.
Most opinion polls show the Yes vote will win with around 60 per cent, but an academic study of Twitter opinions refutes that.
A Guardian Essential poll released today showed that of those polled, 64 per cent voted ‘Yes’ in the survey.
Thirty-one per cent were 'no' voters and five per cent declined to answer.
The ABS estimated last week that more than 12.3 million people, or 77 per cent of eligible votes had participated in the postal survey.
Griffith University professors David Tuffley and Bela Stantic used advanced data analytics which they say have proven accurate at predicting the outcomes of hard-to-call polls, including the 2016 US presidential election.
After running their algorithms, support for Yes came in at 49 per cent.
The Yes campaign has been well supported by a number of high-profile Australians with Julie Bishop revealing that she had voted yes.
The Liberal Party's deputy leader joins Prime Minister Malcolm Turnbull and Opposition Leader Bill Shorten in the Yes camp.
Even if the Yes vote does win, there is no guarantee marriage equality will pass the House of Representatives.
The ABS will announce the result of the survey at 10am on November 15.