05/09/2017 4:56 PM AEST | Updated 05/09/2017 4:56 PM AEST

The Fate Of The Marriage Equality Postal Survey Hangs In The Balance

The High Court set to decide whether the $122 million opinion poll should go ahead.

Supporters of the High Court challenge Lee Gibbens, Sarah Marlowe, Felicity Marlowe and Jacqui Tomlins.

The fate of the Federal Government's plan for a voluntary postal survey on same-sex marriage hangs in the balance today after opponents took the issue to the High Court in Melbourne.

Led by led by chief justice Susan Kiefel, the full bench of the High Court sat in Melbourne on Tuesday to hear two challenges to the Government's same-sex marriage postal survey just a week before forms are set to be sent to households across the nation.

Federal Independent MP Andrew Wilkie, who is leading one of the challenges, has said he is optimistic the High Court will see the urgency in ruling on whether the same-sex marriage postal survey is legal.

Wilkie's lawyers argue the Government doesn't have the power to give the Bureau of Statistics $122 million to run the survey.

"If we succeed in the High Court, there will be enormous pressure on the Government to stop the political game-playing, do their jobs and let MPs vote on marriage equality, Wilkie said ahead of the court appearance on Tuesday.

"Let's not forget that a postal vote wasn't necessary when the Howard Government changed the Marriage Act in 2004 to limit marriage to between a man and a woman and it's not necessary now."

The other case, led by Australians For Marriage Equality, are arguing along the lines of the cost of the survey, the role of the ABS and that Australian Electoral Commission should not be sharing the electoral roll for an opinion poll.

The ABS doesn't normally collect public opinion, instead collecting facts and statistics for analysis.

Ron Merkel QC is representing Melbourne mother Felicity Marlowe as well as Wilkie.

"It's putting to the public a vote on her family unit," Merkel is reported to have told the court.

"We will demonstrate how unique and offensive it is that a personal opinion is being asked on a relationship of this kind."

Marriage equality campaigner Alex Greenwich speaking at a press conference before court.

"We can have marriage equality in the next fortnight, but if we have to be put through this postal vote plebiscite, we are here because we have to make sure it is a legally viable process," Australian Marriage Equality advocate Alex Greenwich said on Tuesday.

"We hope the matter will go back to Parliament and we hope that Federal Parliament will stop delaying, stop dragging this out and finally deliver on the settled will of the Australian people to allow all Australians to marry the person that they love in the country that they love."

The Coalition promised to give Australians a say on whether same-sex marriage should be legal, but its plan was twice rejected by the Senate.

The Government resolved to have a voluntary postal survey, a move that sparked anger from same-sex marriage supporters.

If the challenge is unsuccessful the survey will proceed as planned and advocates will ramp up the case for the "yes" vote.

'Yes' advocates have been warning the so-called "respectful debate" touted by the PM could turn ugly. Indeed, vile homophobic posters, pamphlets and other materials have popped up in Melbourne and Sydney.

An anti same sex marriage poster found in Sydney.

If the challenge is successful, the path is less clear.

Labor says there is no case to spend $122 million on a survey. Opposition Leader Bill Shorten said if the High Court comes down on the side of the postal survey, Labor will campaign for a 'yes' vote.

"We say to LGBTI Australians you are not on your own. This is a weak policy by a weak Prime Minister," Shorten said on Tuesday.

The Greens have said they will push for legislation to be introduced next week for a free vote.

The court is expected to continue hearings on Wednesday, when it will hear submissions from the Commonwealth.