The Turnbull government could be forced to come up with a Plan C on same-sex marriage within weeks, as controversy reins over a new "no" case advertisement.
The High Court will next week hear a challenge to the government's plan for a postal survey on changing marriage laws.
The voluntary survey was Plan B after the Senate blocked the compulsory plebiscite promised by the coalition at the 2016 election.
Constitutional expert George Williams said he expected a quick answer to whether the government had the power to spend money on the survey without legislation having passed parliament.
"It is facing an uphill battle in this case, with its position running counter to line of High Court authority," Prof Williams told the National Press Club on Wednesday.
"Over a series of recent decisions, the High Court has found that the federal government generally requires parliamentary approval to spend taxpayers' money."
The government found the $122 million needed to run the survey by using laws to make an advance payment to the finance minister in circumstances where there is an urgent need for spending and the situation was unforeseen.
Same-sex marriage advocates who are taking the matter to the court argue the spending does not fit the category of either "urgent" or "unforeseen".
"How could this expenditure be said to be unforeseen at the relevant date of May 5, 2017, when the government had a long-standing policy of holding a plebiscite on same-sex marriage?" Prof Williams said.
"Overall, I would be surprised to see the government emerge with a victory."
The advocates are also arguing the survey falls outside the powers of the Australian Bureau of Statistics, which has been asked to roll it out instead of the Australian Electoral Commission which usually runs referendums and elections.
If the survey is found to be unconstitutional, Prime Minister Malcolm Turnbull will face a choice of doing nothing further on same-sex marriage this term, allowing a private member's bill to go to parliament or having another shot at passing the plebiscite bill.
However he has repeatedly promised not to change marriage laws without the Australian people having a say and could face the ire of coalition conservatives if he departs from that.
Meanwhile, Labor has described as hurtful and offensive an ad featuring three mothers highlighting their concerns about how the marriage law changes will effect what is taught and promoted in schools.
One of the women featured on the commercial tells viewers her son had been told he could wear a dress to school next year if he wanted to.
However, the principal of the Victorian high school from which the mother withdrew her children said the offer to students "never happened", Fairfax media reported.
Assistant minister Zed Seselja said the mothers were simply putting the case their ability to object to "fairly radical sex education in schools" would be harder if marriage was redefined.
