Two failed challenges brought against the same-sex marriage postal survey were insubstantial, and misunderstood the limits of the fund used to pay for the poll, the High Court has reasoned in documents published today.

The court delivered its decision with just days to go to the planned start of the survey, but has taken until now to explain its ruling.

A group lead by independent MP Andrew Wilkie and another led by Australian Marriage Equality claimed the survey was not constitutionally valid during a hearing in Melbourne earlier this month.

The main thrust of both arguments was the money for the survey had not been properly appropriated by the Parliament, and thus could not be used legally.

The Government had used a fund set aside for urgent and unforeseen spending to draw the $122 million needed to run the survey.

Federal Independent MP Andrew Wilkie and Felicity Marlowe brought one of the challenges against the survey. ( AAP: Tracey Nearmy )

While the court agreed the Constitution did specify the money had to be properly appropriated, it went on to note that did not mean the Government had overstepped its powers.

Powers 'go back to first act of Parliament'

The challengers argued the passage of the appropriation bill, including the special fund, delegated a power to the Finance Minister that was outside the constitution.

In its judgement the court restated the history of the provision used for the money, going back to the very first act passed by the Australian Parliament.

That act included the power to allocate money to portfolios, with a small portion of money allocated as an Advance to the Treasurer.

The court said that did not allow a treasurer to use unappropriated money, but rather to allocate money from a fund that had already been appropriated.

"What it did was to permit the Treasurer to authorise the delegating to other heads of expenditure, of amounts issued from the Consolidated Revenue Fund under the authority of the Advance to the Treasurer," the reasons said.

The court noted that law had changed little over the years, even after a Finance Ministry was introduced to the mix.

In 2000 there was a complete overhaul, and expenditure could only be made for urgent or unforeseen events.

The judgement for what was urgent or unforeseen was up to the Finance Minister.

The court rejected arguments the Finance Minister was obliged to consider whether such an allocation should be referred for parliamentary approval.

Plaintiffs' standing deemed irrelevant

The court also rejected the suggestion the need for the money must arise from a source external to government.

It found there was no evidence that limitation applies.

In its ruling the court said where the needed expenditure did not exceed the fund then it was available, so long as the Minister was satisfied it met the key criteria of being either urgent or unforeseen.

"That is the reason the amount ... was appropriated in the first place," the court said.

The court had also been asked to rule on whether either of the challengers even had standing to bring the case.

Its judgement said there was no need to make a decision, given the result.

"The merits of the grounds relied on by the plaintiffs in the Wilkie proceeding and the AME proceeding having been fully argued and the court having unanimously reached the conclusion that those grounds were demonstrably without substance," the court found.

"It was similarly unnecessary to determine whether the plaintiffs in those proceedings or any of them had standing in order to reject their claims for relief."

The result of the same-sex marriage survey will be released in November.