The High Court will deliver its decision on the challenge to the Government's plans for a voluntary postal survey on same-sex marriage this afternoon.

Two groups challenged the plan as unconstitutional, taking particular aim at the way the survey would be funded.

The challenge is focused on the $120 million set aside for the survey, which is to be drawn from a fund available for "urgent and unforeseen" matters.

That fund can be used without parliamentary approval.

But challengers argue that the same-sex marriage survey should have been foreseeable and therefore not paid for through that fund.

They say that since the Government earmarked millions of dollars for a compulsory plebiscite on the issue, they believe the Government should have known a "Plan B" option like a postal vote would cost money.

They want the High Court to declare the funding unlawful and rule the survey cannot go ahead.

Lawyers for the Commonwealth struck back at that argument on Wednesday, telling the High Court urgency was a "relative concept".

Commonwealth Solicitor-General Stephen Donaghue told the court the Government had appropriated the money lawfully and there was nothing untoward in the policy.

He said the policy "was developed in circumstances where another policy was put forward and was rejected" - referring to the Government's previous failed attempts at legislating a compulsory plebiscite on the issue.

"A different policy was [then] directed to that end," he said.

He said the Government then made a decision to go with the postal survey.

Decision to use ABS unprecedented: court hears

The court also heard challenges to why the Government was asking the Australian Bureau of Statistics (ABS) to collect opinions, when it is bound by legislation to only collect statistics.

The court heard the decision to use the ABS was "unprecedented".

Previously, Finance Minister Mathias Cormann, the Acting Special Minister of State, told the Senate he was "very confident" the Government had the "constitutional legal authority" for the ABS to collect the data.

"Under [Commonwealth] laws, the Australian statistician and the ABS can request information of all Australians on the electoral roll," Mr Cormann said.

"Indeed, there is express provision in the ABS Act for the ABS to have access to the electoral roll."

The first of two parties challenging the survey is led by Tasmanian independent MP Andrew Wilkie, PFLAG (Parents, Family and Friends of Lesbians and Gays), and Melbourne mother Felicity Marlowe.

The second group involves same-sex marriage advocates Australian Marriage Equality (AME) and Greens senator Janet Rice.

The case has been heard by the High Court during a two-day hearing in Melbourne.

The court is under pressure to deliver a swift verdict, as survey letters are due to be sent to Australian households from next Tuesday.