Two groups have mounted challenges in the High Court, with the postal vote prompting legal questions, fears of hate-filled campaigns and controversy over the millions of dollars it's expected to cost.
The voluntary survey was the government's second plan, after the Senate blocked the compulsory plebiscite promised by the Coalition at the 2016 election.
It wants to hold a non-binding postal survey from next week to gauge public support for changing Australia's marriage law to allow same-sex marriage.
The government found $122 million needed to run the survey under laws permitting an advance payment to the finance minister in circumstances where there is an urgent need for spending and the situation was unforeseen.
But same-sex marriage advocates disputing the matter in the High Court argue the spending is neither "urgent" nor "unforeseen".
Anna Brown, from the Human Rights Law Centre, says above all, the case is about dignity and equality for lesbian, gay, bisexual, transgender and intersex (LGBTI) Australians.
"We are here because we believe and want marriage equality. This postal plebiscite has big question marks around its legal validity. We think it's incredibly important that the legal questions are resolved before we go down the path of having the postal plebiscite, a process that could obviously be challenged down the track as well."
Federal Opposition leader Bill Shorten has reiterated his party's view that the postal survey would be a waste of $122 million.
"We've said if the survey goes ahead, if the High Court upholds it, we will campaign for yes because we say to LGBTI Australians, you are not on your own. This is a weak policy by a weak Prime Minister."
Chief Executive of the Public Interest Advocacy Centre Jonathan Hunyor says parliament alone should debate changing the law.
"We think that parliament needs to make a decision on same-sex marriage and on marriage equality. We think that the government can't try to side step parliament by spending public funds on a process that is unecessary, that is divisive and that is something that we expect parliament to be able to decide in the ordinary course of things."
The court is also looking at whether the survey falls outside the powers of the Australian Bureau of Statistics.
The Prime Minister has tasked the ABS with implementing the survey, instead of the Australian Electoral Commission, which usually runs referendums and elections.
Jonathan Hunyor says that decision is not good for either institution.
"We also have serious concerns about undermining the integrity of institutions like the Australian Electoral Commission and the Australian Bureau of Statistics in a process that is neither really about statistics, nor is it about an election."
For some, like Felicity Marlowe, fighting for marriage equality is a personal campaign.
Ms Marlowe is a lesbian mother of three who has been with her partner, Sarah, for 17 years.
Outside court, Ms Marlowe said she's concerned her children and others from LGBTI families will become collateral damage in what she calls this government's failure to take action in parliament to bring about marriage equality.
"In just the four short weeks since the postal plebiscite was announced, families like mine and trans- and gender-diverse young people have already, without the campaign even starting, have been subjected to vile hate and abuse. I am deeply concerned about the long-term impact of any eight-week public campaign on a postal plebiscite and the impact it will have on the mental health and wellbeing on children like mine, and trans- and gender-diverse young people."
If the survey is found to be unconstitutional, the prime minister will face a number of options, including going ahead with the survey; allowing a private member's bill to go to parliament; attempting to pass the compulsory plebiscite bill; or doing nothing this term.
But he's repeatedly promised not to change marriage laws without the Australian people having a say.
The High Court case continues tomorrow and a decision could be delivered as early as tomorrow afternoon.
