The finance minister has the power to use money from a modest contingency fund to pay for the $122 million same-sex marriage postal survey, the government has told the High Court.
Marriage equality advocates are trying to stop the voluntary 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.
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Commonwealth solicitor-general Dr Stephen Donaghue QC said parliament had passed laws giving the finance minister the power to draw on the advance, which stands at $295 million.
"The purpose of the act is to provide a modest contingency fund to provide for urgent and unforeseen expenditure," he told the full bench of the High Court on Wednesday.
"It is not an appropriation by executive fiat and it doesn't suffer any of the various objectives that the plaintiffs have articulated."
Same-sex marriage advocates 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.
"What we are concerned with in this case is essentially a challenge to the set of provisions to appropriate $122 million," Dr Donaghue said.
He rejected the argument that the section of the act governing the minister's advance is invalid.
"There is plainly no delegation of the appropriation function and so the whole foundation for the attack on the validity of section 10 is removed because parliament has done the appropriating in two different forms."
The solicitor-general said this kind of challenge has not been successful before.
"The plaintiffs are seeking to break new ground," he said.
Dr Donaghue said the minister's advance was merely an earmarking of funds, as the High Court has found in a previous case.
"The appropriation is nothing more than earmarking of funds."
The government also argues the evaluative judgment as to whether something is urgent or unforeseen lies with the minister.
The voluntary survey was Plan B after the Senate blocked the compulsory plebiscite promised by the coalition at the 2016 election.
