The High Court has been told money for the same-sex marriage survey has merely been "earmarked" to be used and it's up to the finance minister to decide what is "urgent" spending.
The government lodged its argument on Wednesday in seeking to fend off a challenge to the survey by marriage equality advocates.
The advocates say the survey breaches the constitution, as parliament did not pass legislation to pay for it and the Australian Bureau of Statistics does not have the legal power to run it.
Constitutional expert George Williams has warned the government faces an uphill battle, arguing a series of recent decisions by the High Court has found the federal government requires parliamentary approval to spend taxpayers' money.
Cabinet minister Simon Birmingham, who supports same-sex marriage, insists the government remains confident it will get the tick of approval from the High Court.
"It would be inappropriate to talk about the hypotheticals of what the court would find," he told ABC radio.
The government says it has properly used 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".
Rather than an "appropriation", which would require legislation, the minister's advance merely "earmarks money for a particular entity and particular purpose", the government said.
As well, the government argues the "evaluative judgement" as to whether something is urgent or unforeseen lies with the minister, Mathias Cormann.
It cited a 1979 report which recommended the criteria for the use of the advance - which stands at $295 million - should be a matter for the finance minister satisfying himself there were urgent and unforeseen circumstances.
In any case, the government said the minister could argue "urgency" because the government did not have a policy for a voluntary survey until August 7 and the ABS needed to start work quickly on the new policy if a result was to be determined by November 15.
While the government accepted that "generalised proposals for a postal plebiscite" had been floated by individual ministers before August, it had not been adopted as government policy.
On the issue of the ABS being able to run the survey, the government submitted the agency was able to collect "data of any kind" including opinions.
A hearing is set down for September 5 and 6.