9.2 million people are estimated to have already voted in Australia's national survey on same-sex marriage, according to figures released by the Australian Bureau of Statistics.
The estimate was released on Tuesday by the ABS in the first of a series of weekly updates of the participation rate in the survey.
It amounts to 57.5% of the eligible voting population.
Deputy statistician Jonathon Palmer, who is in charge of implementing the survey, urged Australians to visit the ABS website or call 1800 572 113 if their form has been lost or damaged and request a new one.
People can request a new form up until 6pm on October 20.
"We hope that this update will serve as a reminder to those who have not submitted their form to do so promptly if they wish to have their say," Palmer said.
The estimated number of responses was as of Friday September 29 and is based on an Australia Post assessment of the number of containers of sorted envelopes.
The ABS also made some adjustments to factor in responses received online and forms that are damaged, invalid, or have their barcodes cancelled.
The forms were first posted out on September 12 and must be returned to the ABS by November 7.
Director of The Equality Campaign Tiernan Brady said on Tuesday that the turnout was good but there would be "no complacency" from the "yes" campaign.
"The 'yes' campaign will continue at full speed because we want every single vote cast so we can have a survey that reflects the values of the Australian people who have long believed it is time for marriage equality," he said.
"It's a simple straight forward question, but its an incredibly important one that will have a profound positive change on the lives of so many Australians."
Spokesperson for the Coalition for Marriage, Lyle Shelton, said the "no" campaign would also "continue tirelessly until the final person casts their vote and the ballot closes".
"The high turnout demonstrates that the Australian public understand the gravity of the decision before us and they know it has consequences for everyone," he said.
"Even so, the statistics released today tell us that one in two Australians are yet to participate in the survey, and we want to make sure everyone has their say."
The figure is in contrast with polling released by The Equality Campaign on Monday indicating 77% of all Australians had returned their forms.
The poll also found that 69% of respondents aged 18-24 had returned their survey forms, while another 17% had said they had filled it out but hadn't made it to a postbox yet.
The poll was conducted by Newgate Research and surveyed 1000 people between September 25-28.