About two-thirds of the nearly 100,000 people added to the electoral roll ahead of the same-sex marriage postal ballot are young voters, according to final figures issued by the Australian Electoral Commission (AEC) on Wednesday.

Between August 8 and 24 the AEC dealt with 933,592 enrolment transactions, 87% of which were changes or updates. Those making changes and re-enrolments were mostly electors aged 25 to 39.

More than 98,000 were added to the roll, of whom 65,000 are aged 18-24.

More than 16 million people are eligible to vote in the voluntary ballot. Voting papers are due to start to go out from September 12, assuming the High Court rules favourably for the government on the constitutionality of the ballot.

The government is confident of the case. But constitutional expert George Williams, dean of the law school at the University of New South Wales, said on Wednesday that “the government is facing an uphill battle” to have the postal ballot upheld “on the state of the law as it currently stands”.

The government is financing the survey out of finance minister’s advance, and having it conducted by the Australian Bureau of Statistics, to get around a lack of parliamentary approval.

But Williams told the National Press Club that over a series of recent decisions the High Court had found government needed in almost all circumstances parliamentary approval for spending.

“This reasoning led the High Court to twice strike down the national schools chaplaincy program. In this case the government, for good reason, has sought on two occasions to get that parliamentary approval but the Senate has denied passage of the relevant law.

"Having been rebuffed, the government is in any event proceeding to spend taxpayers’ money but in doing so has opened up the obvious line of constitutional attack – that it is breaching this clear line of High Court authority.”

Williams said the government’s argument would be that it could rely upon other appropriation legislation. There was an Appropriation Act permitting the advance to be used where the spending was “urgent” and “unforeseen”. But it was hard to show this spending met these criteria, he said.

“Urgency perhaps, but urgency because of the government’s own political priorities but not for other reasons. Unforeseen? We have been debating this for some time. Here it has the appearance of a round peg and a square hole.”

Williams said he would be surprised “if the government emerges with a victory on this”. It would “seem unlikely the court will permit the government a back door means of avoiding the clear line of authority by which the court said you must have explicit parliamentary authorisation for the expenditure of taxpayers’ money”.

On the citizenship cases before the court, Williams was also pessimistic about the government’s prospects of success. He said that “it is difficult to see, if the current law is applied, that any of the seven parliamentarians who will face the High Court are likely to survive that challenge.

"It is hard to see that any of them have taken the reasonable steps that the High Court requires to divest themselves of foreign citizenship.”

Williams said that while the High Court would not revisit any parliamentary votes of someone it found not qualified, “the situation of ministers is far less clear.

"When it comes to ministers the wisest course for them is to refrain from making any decisions or to stand down pending a decision of the High Court.”

Both Barnaby Joyce and Fiona Nash have remained in their ministerial posts. Matt Canavan earlier resigned from the ministry until his parliamentary eligibility is resolved.