Federal Attorney-General George Brandis has predicted same-sex marriage will be legal by Christmas, as the Government prepares to hold a postal vote to gauge Australians' views on the issue.

Key points: If the bill passes the Senate a plebiscite will be held November 25

If the bill passes the Senate a plebiscite will be held November 25 But if it is rejected as expected, a postal vote would start as early as September 12

But if it is rejected as expected, a postal vote would start as early as September 12 The PM says he wants to give "every Australian a say" on the issue

The Government has decided on a non-binding postal ballot if the Senate refuses to approve a compulsory plebiscite.

It would cost $122 million and instead of being run by the Electoral Commission, it would be conducted by the Australian Bureau of Statistics (ABS).

Voters would begin receiving ballots on September 12, ahead of a count in November, but voting would not be compulsory.

The Government will move to revive debate on Wednesday on a bill for a compulsory plebiscite this week, but the Senate is again expected to block it.

Senator Brandis, who is a same-sex marriage supporter, said he "profoundly hopes" the Senate will support the bill, describing a postal plebiscite as "the next best option".

"If the Senate blocks that course of action once again then we will go to a postal plebiscite, because we are going to exhaust every option to ensure the Australian people have their say," he told 7.30.

"My job is to ensure that as many people, as many members of the Australian public participate in this great act of democratic choice."

Senator Brandis said he predicted Australians would vote in favour of same-sex marriage.

"My prediction, and it's only a prediction, is that in the plebiscite, which ever of the two forms it take, more people will vote yes than no," he said.

There are threats of a legal challenge to the postal ballot, which might mean it cannot go ahead on that timetable.

But Prime Minister Malcolm Turnbull said he was confident the postal ballot would survive any constitutional challenge.

He defended the decision to hold a postal plebiscite even though that was not the form of vote he promised at the last election.

Mr Turnbull argued his promise was to give the public a say.

"Strong leaders carry out their promises, weak leaders break them," Mr Turnbull said.

"I'm a strong leader. I made that promise again and again… you heard me say again and again that every Australian will have a say on this issue."

Sorry, this video has expired Malcolm Turnbull has defended the decision to hold a postal plebiscite

'People's hopes dashed by weak PM': Shorten

Labor leader Bill Shorten attacked the Coalition's decision saying "the hopes of people who want to be able to marry the person they love have been dashed by a weak Prime Minister and the right wing of the Liberal Party".

He called it a colossal waste of money and time.

Deputy Labor leader Tanya Plibersek dismissed the idea of the ABS running the postal ballot.

"The same people that brought you the census debacle. I mean, it's not very reassuring is it?" Ms Plibersek told Sky News.

She noted that some MPs have said they would ignore the result.

Bill Shorten, Penny Wong and Tanya Plibersek spoke about Labor's stance on the same-sex marriage debate outside Parliament House. ( ABC News: Jed Cooper )

Tasmanian Liberal senator Eric Abetz said he would be guided by the way people in his state vote, rather than the overall result.

"To be clear, it is impossible to determine whether a parliamentarian should adhere to their electorate, their state or to the national vote and in those circumstances, all parliamentarians should be given a degree of leeway to adhere to how they can best represent those who elected them," Senator Abetz said.

"If electors feel that their representative has not represented their views well, they can then take action at the next election."

If either form of plebiscite happens, Parliament would have two sitting weeks this year to vote on the Marriage Act and potentially legalise same-sex marriage.

Senator Matthias Cormann said if the plebiscite result was in favour of same-sex marriage, Coalition MPs would have a conscience vote on a private members' bill.

Labor has a conscience vote until the next election and, after that, members of the ALP would be bound to vote to allow same-sex couples to marry.

Campaigners for same-sex marriage are confident the law will be changed if Coalition MPs are given a conscience vote on a bill.