With just two weeks left in the same-sex marriage survey, young people have been urged to get their votes in or they won't be counted.
Thousands of people who attended a rally in Melbourne on Sunday in support of same-sex marriage were told to get on to their friends and family to ensure they voted.
Data shows people aged 18 to 35 haven't returned their ballots in the same numbers as older age groups.
"Get that ballot out of your gym bag, get it from your parents' house, make sure you put it in the post, because unless you've posted you haven't voted," equality campaign co-chair Anna Brown told reporters on Sunday.
She urged them to get ballots in before October 27, to make sure they arrive before the November 7 cut-off date.
"There's only a week left to get that ballot in the mail. This is the home stretch," she said.
The rally saw thousands march through the rain in Melbourne's CBD to a free concert featuring Spiderbait.
Ms Brown said a "yes" vote would go a long way to promoting acceptance and celebration of Australia's LGBTQI community.
More than 70 per cent of forms have already been returned, and Equality Campaign executive director Tiernan Brady is confident the majority of Australians will back changing the Marriage Act.
But Australian Conservatives leader Cory Bernardi doesn't accept there will be an overwhelming 'yes' vote to the same-sex marriage postal survey.
He believes people are feeling under pressure from political correctness and are saying they voted "yes", even if they have voted "no".
"I'm not even conceding defeat quite frankly. I think there are a great many people who are deeply concerned about the potential consequences of changing the marriage act," Senator Bernardi told Sky News on Sunday.
The Australian Bureau of Statistics will release results of the survey on November 15.
