Same-sex marriage advocates launched legal action in Australia's highest court Tuesday against a controversial government plan for a postal vote on the issue, calling it divisive and harmful.
Polls indicate popular support in Australia for marriage equality, but a standoff has dragged on for more than a decade amid political wrangling over the best way to decide the matter.
After parliament's upper house, the Senate, last year rebuffed plans for a national plebiscite involving 15 million people, the government in August said a voluntary postal ballot would be held.
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.
They favour a free parliamentary vote, with MPs not restricted by party policy.
Anna Brown, from the Human Rights Centre which is representing advocacy group Australian Marriage Equality and Greens senator Janet Rice in the legal action, said she was confident the Melbourne court would rule the vote was invalid.
"The postal plebiscite is unnecessary and is already proving divisive and harmful. LGBTI (lesbian, gay, bisexual, trans and intersex) groups strongly oppose the plebiscite and so do we," she said.
"Telling one group of people that their rights have to be decided by a public vote sends a terrible message."
It is one of two legal challenges being heard by the court, with the other led by Independent MP Andrew Wilkie, PFLAG (Parents, Family and Friends of Lesbians and Gays), and Melbourne mother Felicity Marlowe.
Opponents claim the survey falls outside the powers of the Australian Bureau of Statistics, which has been hired to conduct the poll.
They also say Canberra exceeded its executive authority by earmarking Aus$122 million (US$96 million) to carry it out without parliamentary approval.
The government argues it can authorise using the cash under laws that allow it to green-light "urgent" and "unforeseen" spending.
Ballot papers were due to be sent out next week.
Prime Minister Malcolm Turnbull supports marriage equality, but he is battling with right-wing members of his own Liberal Party, who are against legalising such unions.
Marlow, a lesbian mother-of-three, said since the postal survey was announced, she has suffered "vile hate and abuse".
"Our ability to nurture and love and care for our children has been questioned already on social media, on posters, on flyers, on advertisements in the newspapers," she said.
If the ballot goes ahead and a majority of Australians vote "yes", the government would hold a free vote in parliament on the issue, with MPs not bound by party policy or the postal ballot's result.
If there is a "no" outcome, there would be no parliamentary vote.