The Australian Bureau of Statistics estimates 57.5 per cent of ballot papers in the same-sex marriage postal survey have been returned so far.
More than 16 million ballot papers have been mailed to Australian voters.
The survey is asking Australians whether they believe the law should be changed to allow same-sex couples to marry.
Both sides of the debate about whether to change the marriage laws are using the figures to urge people to return their survey forms.
No campaign spokesman Lyle Shelton said the ABS estimate showed nearly half of all Australians have not yet sent back their survey.
"We want to make sure everyone has their say," he said.
But he added the high turnout "demonstrates the Australian public understands the gravity of the decision before us and that they know it has consequences for everyone".
Anna Brown from the Yes campaign also urged those who have not sent their forms back to act.
"Whether it is in the gym bag or on the kitchen table, people simply need to get their ballots in as soon as possible," she said.
Greens senator Janet Rice said it was heartening that 57.5 per cent of votes had been returned.
"It is a good sign… It just goes to show that Australians care deeply about this issue," Senator Rice said.
Federal Cabinet Minister Darren Chester said the result so far is "a complete vindication of the government's position, that the Australian people did want to have their say".
"They have voted early and they have made their opinions known and I think that is a good result," Mr Chester said.
"I am strongly of the view that right across Australia there will be a resounding Yes vote.
"I think some of my colleagues will be surprised… I think every electorate in Australia is going to vote Yes and I think some people will be surprised by that in their own electorates,
"I could be wrong but that is my view."
The result will be announced on November 15.