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.