CONFIRMATION today of a postal ballot on same-sex marriage started a race to sign up thousands of “missing” young people who could swing the result if they are registered to vote.

There are close to 300,000 people aged 18 to 24 who have yet to take up their right to vote, and campaigners believe many of them would support marriage equality.

The Government this morning was rebuffed in the Senate when it attempted to revive legislation, stalled since last November, for a $270 million plebiscite on marriage laws.

The 31-31 vote meant there was no majority needed to take the legislation to the next step.

Prime Minister Malcolm Turnbull has said a voluntary, non-binding, $122 million postal ballot — run by the Australian Electoral Commission and the Australian Bureau of Statistics — will be held instead of the full plebiscite.

The AEC estimates 810,904 people are “missing” from the rolls.

And as of last March, just 86.7 per cent of 18-24 year olds were registered. By one calculation, that means around 274,000 young people will have to move quickly if they want to vote in the SSM postal ballot.

Just 71 per cent of 18-year-olds are on the rolls, 83.7 per cent of 19-year-olds and 93.3 per cent of those aged 25 to 39.

Mr Turnbull believes there could be a vote in Parliament on changes to the Marriage Act by late November.

To meet that deadline, ballot papers will have to be delivered to households by September 12 and the outcome of the vote announced by November 15.

That leaves just over four weeks for unregistered voters to get themselves on the roll, a process which can take two weeks to complete although the actual filling out of a form can be done quickly and easily.

While the poll could be delayed by threatened constitutional challenges to the High Court, campaigners will still be aiming to reach young voters who are yet to enrol.

Former Prime Minister Tony Abbott repeated he would be voting against SSM, but his sister made clear on Twitter she would be supporting it.

“Talk to everyone. Tell them what this means to people. Tell them it’s a simple thing: it’s about fairness. Ask them to vote yes x,” urged Christine Forster on Twitter today.

And broadcaster Alan Jones also took up the yes case.

“Re Gay Marriage. Love is a very elusive thing. If 2 people find love we shouldn’t be making judgments about it or getting in the way,” tweeted Mr Jones today.