Lawyers for the Commonwealth have struck back at arguments that funding for the same-sex marriage postal survey was not urgent or unforeseen, telling the High Court urgency is a "relative concept".

The Commonwealth Solicitor-General Stephen Donaghue slammed the argument as a "misconception" and defended the survey as appropriate use of a government contingency fund.

The Federal Government's same-sex marriage postal survey is being debated in the High Court for a second day, with the focus shifting to the Commonwealth case.

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" items, which could be used without parliamentary approval.

Mr Donaghue told the court the level of urgency required a judgement to be made about relative priorities.

"Urgency is a relative concept," Mr Donaghue said.

He said it was not an error for the Finance Minister to decide to draw the money from the contingency fund for the survey.

Mr Donaghue rejected criticism the money should have been appropriated with a vote from the Parliament and told the High Court it was not usual to turn to the Parliament for such funding.

"If that was the case one would expect to see very many special appropriation acts," Mr Donaghue said.

"But in fact one sees nothing of the kind.

"Parliamentary practice does not bear out the idea one needs to go back for special appropriations."

Two groups are opposing the Government's same-sex marriage survey and told the High Court yesterday it was unconstitutional and there was nothing urgent or unforeseen about the survey.

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

Nothing untoward about survey, Government argues

Mr Donaghue told the court there was nothing untoward about the Government's pursuit of a postal survey on same-sex marriage.

"This is a policy developed in circumstances where another policy was put forward and was rejected," he said.

Mr Donaghue was 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.

When the High Court begun hearing the Commonwealth's case, Mr Donaghue told the court the Government's plan was not unconstitutional.

He said one of the groups, led by MP Andrew Wilkie, had attacked the appropriation of the money directly and the other group, represented by Australian Marriage Equality (AME), was attacking the law itself.

"The AME is challenging the amendment of the legislation," Mr Donaghue said.

Concerns survey passes judgement on people's sexuality

Justice Geoffrey Nettle also asked about concerns raised by those in the case that the survey was passing judgement on their sexuality.

Mr Donaghue said the mere fact of the survey was not the source of that concern.

"Any aspersion on their relationship is a consequence of the current law, the question is seeking to remove that," he said.

The survey was announced by the Government last month, after the Senate rejected a compulsory plebiscite on same-sex marriage twice.

The challengers are also fighting against the use of the Australian Bureau of Statistics (ABS) to run the opinion survey, when it is bound by legislation to collect statistics.

The High Court can take as long as it likes to hand down a decision.

But survey letters are due to be sent to Australian households from early next week, adding pressure on the verdict to be delivered quickly.

Finance Minister Mathias Cormann told ABC the ABS had began printing the survey forms, instruction letters and envelopes.