That's a stark contrast with the 86 per cent predicted for the marriage equality vote.
The country is now voting on whether marriage rights should be extended to same-sex couples in a non-binding postal vote.
AME research shows 69 per cent of 18-24-year-olds have already voted while more than 80 per cent of over-65s have voted.
If you are yet to return your vote to the ABS, remember the cutoff date is November 7th.
Polling had shown a drop in support for equal marriage as anti-LGBT activists began a divisive campaign based on tactics developed by evangelicals in the USA, attempting to conflate the marriage vote with transgender issues and LGBT sex education.
Jonathon Palmer, the Deputy Statistician and Taskforce Lead, said the weekly updates would hopefully see more people "have their say".
The ballot paper will read: "Should the law be changed to allow same-sex couples to marry?"
The Coalition for Marriage campaign dismissed Monday's poll outright.
Liberal MP Kevin Andrews was sceptical of the polling
The deadline for the return of ballots is November 7, giving a voting period of almost two months. The results of the poll will be declared on November 15.
The Equality Campaign's Alex Greenwich said a further 17% of people in that age group said they had filled out their forms but were yet to post them.
Acting Special Minister of State Mathias Cormann, who has responsibility for the survey, said the turnout showed Australians were "embracing [the] opportunity to have their say on marriage".
Shorten said Australians support marriage equality and "just want the matter finished".
"The high turnout demonstrates that the Australian public understand the gravity of the decision before us, and that they know it has consequences for everyone", said Lyle Shelton, spokesman for the Coalition for Marriage.
"Today the Campaign releases new TV and Radio advertisements calling supporters across the nation to get out and vote".
A separate poll of 1000 Australian women aged 25 to 44 released this weekend by The Equality Campaign found 75% declared they were voting "yes" in the survey.
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