The government should not have bypassed parliament by giving the finance minister the power to fund the postal survey on same-sex marriage, the High Court has heard.
Marriage equality advocates trying to stop the postal survey argue there was no urgent need for the advance to the minister used to fund the $122 million survey nor was the situation unforeseen.
The government bypassed parliament when there was no urgent need to do so, Ron Merkel QC told the full bench of the High Court.
"There was no urgent need because the government was quite able to put the appropriation for the survey to the House or the Senate, but it didn't occur," the former Federal Court judge said on Tuesday.
"On the facts of this case, no minister acting reasonably could have concluded in accordance with the law that this was an urgent need."
The voluntary survey was Plan B after the Senate blocked the compulsory plebiscite promised by the coalition at the 2016 election.
Finance Minister Mathias Cormann stated in an affidavit to the court that the options being canvassed by cabinet before the second Senate knockback in August did not include a postal survey.
Advocates' barrister Katherine Richardson SC said the minister knew his colleagues were considering an alternative means of a plebiscite, seeking the views of 16 million people on the electoral roll.
"The minister would be taken to know the alternative means they were contemplating must involve significant expenditure because what they were contemplating was a plebiscite of 16 million people," she said.
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.
Rather than an appropriation that would require legislation, the government has said the minister's advance merely earmarks money for a particular entity and a particular purpose.
Mr Merkel said if the government was correct, there would be no need for special appropriations as occurred with the Northern Territory intervention.
The advance to the finance minister would be a standing appropriation, he said.
"That is a quantum leap that has no historical foundation for it and in fact the constitution stands directly against it."
Mr Merkel said his client, independent MP Andrew Wilkie, has been denied the opportunity to vote in parliament on the issue.
"The people have given Mr Wilkie and other members of parliament the constitutional role in ensuring that monies are appropriated in accordance with the law."
The two groups of same-sex marriage advocates are also arguing the survey falls outside the powers of the Australian Bureau of Statistics.
Barrister Kathleen Foley said the statistician was being asked to conduct a postal survey and get yes and no answers, rather than provide a collection of views from the Australian population.
Justice Patrick Keane noted participation is voluntary.
"If someone doesn't want to participate or express a view one way or the other, they can just throw it in the bin," he said.