More than 12 million Australians have already participated in the Turnbull government's same-sex marriage postal survey, new polling suggests.
With a month still to run until the survey closes, the polling conducted for The Equality Campaign finds a massive 77 per cent of the nation's 16 million enrolled voters have returned their forms.
That includes 69 per cent of younger voters - aged 18 to 24 - who are overwhelmingly expected to support same-sex marriage. That's more than double the number of 18- to 24-year-olds who took part in the Howard government's 1998 postal ballot.
A further 17 per cent of people in this age group say they have filled out their forms but have yet to put them in the post, the survey of 1000 people found.
A separate Sky News ReachTEL poll released on Monday found 64 per cent of people had returned their ballot and voted "yes". Just 16 per cent had returned their ballot and voted "no".
But the Equality Campaign polling also contains some potential good news for the "no" campaign, with 80 per cent of over-65s - who are believed to be more likely to support a more traditional view of marriage - also saying they have already returned their forms.
The results of the poll - conducted last week by Newgate Research - came a day before the Australian Bureau of Statistics is due to release its official participation figures. It will release participation figures once a week between now and the November 7 deadline.
If the polling prove accurate, it will smash Ireland's 60.5 per cent voter turnout in its 2015 same-sex marriage referendum.
Tiernan Brady, director of The Equality Campaign, said he was delighted with the turnout of young Australians in particular.
"We got a taste of the enthusiasm of young Australians when record numbers enrolled on the electoral roll so that they could vote "yes" for their friends, family and workmates," he said.
"Young people, along with the majority of Australians understand this about real lives, real people and their dignity."
"No" campaign spokesman Lyle Shelton - from the Australian Christian Lobby - said he would be surprised if the participation rate was so high.
"I suspect there are still millions and millions of Australians who are yet to cast their vote," he told Sky News.
Mr Shelton said he would welcome a high participation rate, saying anything over 60 per cent would be credible. But the "no" campaign will keep fighting "right down to the wire", he said.
"This is a David and Goliath battle, we don't have the NRL giving free publicity during grand finals, so we need all the help that we can get," he said.
New Australian Electoral Commission figures released in recent days confirmed there was a 98,000-strong surge in enrolment ahead of the postal survey, with the number driven by 18- to 24-year-olds, women and inner-city residents.
There were nearly 1 million "enrolment transactions", including changes to details.