The full bench of the high court will hear challenges to the proposed same-sex marriage postal vote next month, just days before ballots are sent to Australian homes.

Chief Justice Susan Kiefel set down the matter to come before the court on 5 and 6 September. The ABS has given an undertaking it would not send out voting forms any time before 12 September.

A directions hearing into the challenges will be heard next week.

Two separate challenges have been brought against the postal vote: one by independent MP Andrew Wilkie, marriage equality advocate Felicity Marlowe, and Parents and Friends of Lesbians and Gays Brisbane; the second by Australian Marriage Equality and Victorian Greens Senator Janet Rice.

Both argue that the government is acting unconstitutionally in appropriating $122m for the vote and ordering the Australian Bureau of Statistics to run the postal survey.

Keifel CJ said the court was seized of the urgency of the matter. The government has nominated 12 September as the beginning of the postal survey mail-out.

“It goes without saying … that the timetable is incredibly tight and the parties will need to adhere to it scrupulously,” Kiefel CJ said.

Outside court in Sydney, Anna Brown said there were important questions about the lawfulness of the postal survey.

“It is appropriate for the High Court to consider them,” Brown said.

“All Australians should have the same opportunities for love, commitment and happiness. Australians don’t want more excuses or delaying tactics – we want marriage equality.”

The challenges will argue the government cannot appropriate $122m for the postal survey as either ordinary government business, or as “unforeseen circumstances”.

Submissions to the court say the government first sought legal advice on a postal vote as long ago as March, months before the May budget, and has not passed legislation to appropriate money for the postal vote.

Australian Marriage Equality co-chair Alex Greenwich said he was encouraged the High Court had chosen to hear the case, and quickly, reflecting the importance of the issue to the Australian people.

Greens Senator Janet Rice, party to one of the challenges, said the government’s plebiscite had been twice rejected by the Senate, and that the government was now seeking to subvert the will of the parliament by proceeding with a plebiscite without parliamentary approval.

Also on Friday, the Turnbull government moved to address various concerns about the integrity of the postal survey.

Acting special minister of state, Mathias Cormann, reached out to Labor and the Greens offering to extend electoral law provisions banning misleading information, fraud, bribery and intimidation to protect the survey.

Concerns have been raised that silent electors, who can vote in ordinary elections but whose addresses are not recorded on the roll, could be disenfranchised. Cormann said the government was committed to ensuring “all Australia on the election roll, including silent electors, have the opportunity to have their say”.

“The ABS will make further announcements in relation to this as soon as arrangements have been finalised.”

Labor also questioned whether the government had accidentally allowed 16 and 17 year olds to vote in the poll. According to the relevant legislative instrument made by treasurer, Scott Morrison, people who have made “valid applications for enrolment on the Commonwealth electoral roll before the end of 24 August 2017” will be entitled to vote.

Under the Electoral Act Australians aged 16 and 17 are allowed to make a “claim for enrolment” which results in provisional enrolment. Labor believes this may mean 16 and 17 year olds are entitled to vote in the same-sex marriage survey.

It comes as the Equality Campaign, run by Australian Marriage Equality, launched on Friday, just days after AME co-chair Alex Greenwich refused to rule out a boycott.

The executive director of the Equality Campaign, Tiernan Brady, said if the postal survey goes ahead “we are in this to win this”.

“We owe it to the hundreds of thousands of LGBTI people across Australia who want the same dignity as everyone else and ... to the vast majority of Australians who have long felt that marriage equality should be the law of our land,” he said.

Brady urged people to enrol to vote, describing it as “part of the unfair process” that Australians, especially young people, would have just 12 days to enrol or update their enrolment.

Greenwich said that marriage equality advocates had a “duty of care and responsibility” to campaign and achieve marriage equality by the end of the year.