Gay rights advocates have argued in Australia's highest court that the government does not have the power to bypass parliament to pay for a US$122m public survey on whether gay marriage should be legalised.
The advocates want parliament to decide the issue without consultation with the public and argued in the Australian High Court that the government does not have the constitutional power to start the postal survey next week.
The government will argue its case tomorrow.
Opinion polls show that most Australians want same-sex marriage legalised, but many advocates question how representative of Australian attitudes the postal survey would be.
Seven judges are hearing two similar cases simultaneously in the city of Melbourne. The judges could rule on the validity of the survey as early as Wednesday and prevent ballots being posted to voters from September 12.
The survey is the second choice of prime minister Malcolm Turnbull's conservative government, which had promised a rare, compulsory vote known as a plebiscite.
The senate refused to approve the $170m that the vote on November 25 would have cost, so the government is pushing for a voluntary postal vote without senate approval at a cost of AUS$122m.
If a majority wants marriage equality, parliament would be allowed to decide the issue by December, but some legislators have said their votes in parliament would not be swayed by public opinion, raising questions about why the public is being surveyed.
Lawyer Ron Merkel told the court today that the government's power to fund such a vote without senate approval can only be used in unforeseen emergencies.
He said there is no urgent need to hold the postal survey.
If the court stops the survey, legislators in the minor Greens party are pushing for a bill to legalise gay marriage to be introduced to the senate next week.