Mike Keating, who led the Department of the Prime Minister and Cabinet under Paul Keating, says the government is "ignoring all proper conventions" by taking the money from the 'Advance for the Minister of Finance' in an "entirely inappropriate use" of the fund, which is meant only for urgent and unexpected circumstances.

But the government, which is facing a challenge in the High Court to the spending, insists it is on solid legal and constitutional grounds in giving the money to the Australian Bureau of Statistics to conduct the survey.

Mr Keating says the fund is now being abused to thwart the Federal Parliament and Commonwealth auditors should scrutinize the Coalition's conduct.

The High Court will determine in early September, when it considers two cases challenging the validity of the postal survey, whether the government has acted within the law by plucking the $122 million cost of the vote from the advance funds.

The Coalition has used the fund just once before since coming to government in 2013, giving $101 million to the Electoral Commission last year to pay for Senate voting reform.

By contrast, the Rudd/Gillard governments routinely dipped into the advance, Finance Department documents show, with 32 raids on the fund between 2008 and 2013, spending more than $890 million in total.

But Mr Keating, who led the Finance Department from 1986 until 1991, is calling on the Auditor-General to look at the funding of the postal vote and to consider tighter controls around the advance so that it cannot be abused to bypass the will of the Parliament.

Writing in the Pearls and Irritations public sector blog, the former top bureaucrat says the department, which should be consulted by government about spending from the fund, would not have approved the postal vote spending from advance funds under his watch.

"The Government has decided to ignore all conventions about proper procedure, and by-pass the need for parliamentary approval by drawing on the Advance to the Minister of Finance to pay for a more limited postal plebiscite," Mr Keating said.

"It would seem that the government is seeking to use the Advance to the Minister of Finance to avoid having to gain parliamentary approval of its actions.

"To me that seems to be an entirely inappropriate use of the Advance."

Mr Keating echoed the views of another former senior Finance official, Stephen Bartos, who said in early August that using the advance to pay for the postal vote would be "really out, in terms of the rules".

"It would be incredibly doubtful that you could argue that a postal plebiscite was urgent and unforeseen," Mr Bartos said.

Neither Finance Minister Mathias Cormann or his department would answer questions on Friday about what advice was sought or provided about the funding of the postal vote, nor did they respond to a request to release any advice.

But the minister said he was confident that the legal foundations of the controversial postal vote were sound.

"The government is confident that we have the constitutional power and statutory authority to proceed with the Australian Marriage Law Survey as proposed," Senator Cormann said.

"This is now a matter for the High Court."