Support for a "no" vote has risen as the same-sex marriage postal survey gets under way.
Turnout among "yes" voters could be very low as well, research for Fairfax Media shows, with a marked drop among those aged 18 to 34.
Years of research has shown most Australians support same-sex marriage and the "no" camp has been seen as the underdog.
But the research shows more people are undecided on how they will vote.
