The High Court is to consider a bid by marriage equality advocates to stop the government’s postal survey.

The voluntary survey was Plan B after the Senate blocked the compulsory plebiscite promised by the coalition at the 2016 election.

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”.

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.

Constitutional expert George Williams says Malcolm Turnbull has an uphill battle on his hands to save the survey because the court has previously found governments need parliamentary approval to spend taxpayers’ money.

If the survey is found to be unconstitutional, the prime minister will face a choice of trying to fix the problem with legislation and going ahead with the survey; allowing a private member’s bill to go to Parliament; having another shot at passing the compulsory plebiscite bill, or doing nothing this term.

However, Mr Turnbull 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.

The case will be heard before the full court in Melbourne over two days on Tuesday and Wednesday.

The ABS has committed not to send any survey forms until the judgement is known.