The Turnbull government will endure a nerve-wracking wait on Thursday with the High Court to decide the fate of the same-sex marriage survey in the early afternoon.

Following two days of hearings, Chief Justice Susan Kiefel on Wednesday said the court would reveal its findings at 2.15pm on Thursday, meaning the decision will be handed down as the government and opposition go head-to-head during Question Time.

Given ballot papers are set to be sent out on September 12, the court decided to expedite its decision. It will publish more detailed findings at a later date.

On Wednesday, the High Court heard from Commonwealth solicitor-general Dr Stephen Donaghue QC, who defended the constitutionality of the survey to be conducted by the Australian Bureau of Statistics.

The government has used an advance fund earmarked for “urgent” and “unforeseen” spending to provide the Australian Bureau of Statistics with the money it needs to conduct the vote.

“The purpose of the act is to provide a modest contingency fund to provide for urgent and unforeseen expenditure,” Dr Donaghue told the High Court on Wednesday.

He argued the Finance Minister had the authority to appropriate the $122 million needed to fund the survey and that it was up to the minister, Mathias Cormann, to determine if the cost was “urgent” and “unforeseen”.

“What we are concerned with in this case is essentially a challenge to the set of provisions to appropriate $122 million,” he said.

Senator Cormann’s move was in keeping with the practice of previous governments, Dr Donaghue argued, adding that governments were entitled to utilise the fund for nationally important causes that arose outside the budget cycle.

Dr Donaghue also sought to challenge the plaintiffs’ standing in the case – that is whether they would be directly affected by the decision taken by the government.

He disputed the argument put forward by lawyers for the plaintiffs –which includes Melbourne lesbian mother Felicity Marlowe – that people would be voting on the legitimacy of her family unit and that this would therefore offend Ms Marlowe.

The High Court has been considering the challenge to the postal survey for two days. Photo: AAP

On Tuesday, lawyers for the two separate challenges to the survey argued it was not “urgent and unforeseen”, noting that government ministers had flagged a possible postal plebiscite before the federal budget.

They have also argued that the ABS, which is legally authorised to collect statistics, is not able to conduct the survey under the existing legislation, and that it does not constitute “ordinary annual government services” as is required.

The government pushed forward with the postal vote without legislation after the Senate twice rejected its preferred option of a compulsory attendance plebiscite.

The opposition has opposed both plebiscite models, but has said it will campaign for a ‘yes’ vote if the survey is ruled legal.

The government has not explicitly stated what it will do if the survey is ruled unlawful, though some conservative Coalition MPs have already said that the PM should not facilitate a free vote.

The postal survey will ask voters the question: “Should the law should be changed to allow same sex couples to marry?”

If it goes ahead, the result will be announced on November 15.

-with AAP