The full bench of the High Court will hand down their decision in Melbourne at 2.15pm on Thursday, during Question Time in federal parliament.
After the Senate, the upper house of the Australian parliament, rejected plans for a national plebiscite on the matter involving 15 million people in 2016, the government said in August this year that a voluntary postal ballot would be held.
Co-Chair of Australian Marriage Equality Alex Greenwich, Director of Legal Advocacy, Human Rights Law Centre Anna Brown, Public Interest Advocacy Centre Chief Executive Officer Jonathon Hunyor and Director of Rainbow Families Victoria Felicity Marlowe outside the High Court this morning.
Both options are strongly opposed by gay marriage advocates, who argue that a national vote is expensive and will subject gay people and their families to hate speech. The Coalition will be back to square one with conservatives in the party refusing to have a conscience vote until a plebiscite occurs and marriage equality advocates pushing for the free vote. "This postal plebiscite has big question marks around its legal validity".
"Telling one group of people that their rights have to be decided by a public vote sends a bad message", said Anna Brown, a lawyer involved in one challenge.
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The government's representative, Dr Donaghue, was also forced to respond to arguments that the use of the ABS was unlawful because the government was asking for an opinion, not a statistic.
The government unlocked A$122m for the survey through an act allowing it to fund initiatives determined to be "urgent and unforeseen".
Ballot papers were due to be sent out next week.
If either of these challenges is successful, the vote will not go ahead, but Prime Minister Turnbull does not appear to be anxious.
"The government is exceeding its power by trying to conduct the postal vote, exceeding its power by trying to spend $122 million without parliamentary approval", he told ABC radio. MPs then would not be restricted by party policy or the results of the ballot.
Barrister Ron Merkel QC said independent MP Andrew Wilkie has been denied the opportunity to vote in parliament on the issue.