Marriage equality advocates and the Turnbull government will soon find out who has won the legal battle over the same-sex marriage postal survey.
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.
The decision will determine whether the Turnbull government has to come up with a Plan C on same-sex marriage, having decided on a Plan B voluntary survey after the Senate blocked the compulsory plebiscite promised by the coalition at the 2016 election.
Two groups of same-sex marriage advocates are trying to stop the postal survey, arguing the government should not have bypassed parliament in funding it.
The government found the $122 million 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.
The challengers argue the spending was neither urgent nor unforeseen, two key requirements for advancing money from the pool of funds that can be used without parliamentary approval.
The first hurdle for the two groups of advocates, who include independent MP Andrew Wilkie and Greens senator Janet Rice, is for the High Court to rule they have special interest in the case because they are directly affected.
They will lose the case if the judges decide the plaintiffs do not have standing to bring it to the court.
The remaining questions before the seven judges centre on the constitutional validity of the expenditure of funds on the postal survey and the minister's power to make the advance.
Should they succeed, the advocates are seeking injunctions to stop the survey.
The possibility of the Australian Bureau of Statistics funding the survey out of its $340 million budget allocation, rather than the advance, has been raised although the plaintiff's barristers are also seeking injunctions to prevent that.
The survey forms are due to be mailed from Tuesday, September 12.
