Hundreds of young people have enrolled in Wannon and Mallee to vote in 2017 same-sex marriage postal survey

The number of new young voters includes more than 600 aged between 18 and 19 on August 24.

The voters all contacted the Australian Electoral Commission to add their details to the roll between August 8-24 when the postal survey was announced.

All those who enrolled will also be legally required to vote in upcoming federal elections as well as being eligible for the option postal survey run by the Australian Bureau of Statistics.

The voluntary survey that asks ‘Should the law be changed to allow same-sex couples to marry?’ must be posted back to the ABS by October 27 in order to be counted.

Map of how many 18-year-olds enrolled in each federal electorate 

Wannon, represented by Liberal MP Dan Tehan, gained 473 new voters of all ages while Mallee, represented by Nationals MP Andrew Broad, gained 536.

Wannon now has 396 new voters aged under 30, including 291 aged 18.

Mallee now has 463 new voters aged under 30, included 350 aged 18.

The new young voters represent about 0.39 per cent of all voters in Wannon, which Mr Tehan held with a margin of 10.1 per cent at the 2016 election.

The new young voters represent about 0.46 per cent of all voters in Mallee, which Mr Broad held with a margin of 37 per cent at the 2016 election.

Mr Broad has pledged to vote in parliament for or against same-sex marriage on the basis of the vote results in Mallee.

Mr Tehan has pledged to follow the national result when he votes on legislation.

Both Mr Tehan and Mr Broad, like all MPs and senators, are under no legal obligation to do anything as a result of the postal survey.

The ABS announced on Tuesday that, as of September 29, it had received 9.2 million completed survey forms, representing 57.5 per cent of the total mailed out.

More than 12 million Australians have already participated in the federal 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.

-with Adam Gartrell, Sydney Morning Herald