The High Court has agreed to hear a challenge to the Federal Government's same-sex marriage postal ballot on September 5 and 6, with a directions hearing to be held on Thursday.

The challenge will be heard a week before the postal vote is due to begin, with letters slated to be posted on September 12 to November 7, and the outcome to be announced on November 15.

Two separate parties applied for a temporary injunction preventing the postal vote but told a hearing this afternoon that they would be prepared to put their bid on hold given the case was being urgently heard.

In Melbourne, Federal independent MP Andrew Wilkie joined PFLAG (Parents and Friends of Lesbians and Gays) and Felicity Marlowe, who is raising three young children with her partner Sarah Marlowe, in challenging the case.

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Represented by the Public Interest Advocacy Centre, they are arguing Finance Minister Mathias Cormann does not have the authority to appropriate funds for the postal vote.

They are also arguing that the Australian Bureau of Statistics has unconstitutionally been directed to seek an opinion rather than "statistical information".

Outside the court, Mr Wilkie said High Court Chief Justice Susan Kiefel's decision to hear the case before a full bench was a win for people in the community who had concerns about the non-binding, voluntary postal vote.

"I've always felt that our case has merit. I think it is self-evident that the Federal Government is exceeding its powers by trying to conduct a postal vote," he said.

"It's exceeding its powers by thinking it can spend $122 million of public funds without parliamentary approval.

"It's self-evident that it is improper to ask the bureau of statistics to effectively act unlawfully by contravening the act which regulates them."

Government 'must wear consequences of bad decision'

Mr Wilkie said both parties had agreed not to go ahead with the injunction because the matter would be heard before September 12.

"We didn't need to pursue it because the Government has made an undertaking to the High Court of Australia that no papers will be distributed before [September 12]," he said.

"If the Government decides to spend any public funds between now and when this matter is resolved by the High Court ... that will be the Government's decision and the Government will have to wear the consequences of a bad decision like that if they lose in the High Court."

Alex Greenwich, from the Australian Marriage Equality group, said supporters must re-double efforts to campaign for same-sex marriage.

"Regardless of the outcome we will always continue to campaign and fight for marriage equality," he said.

Felicity Marlowe (left) and her partner of 17 years, Sarah Marlowe, are challenging the same-sex marriage postal survey. ( AAP: Tracey Nearmy )

"Should we have to deal with a postal vote plebiscite being inflicted upon us, we have a duty of care and responsibility to make sure we campaign for marriage equality, to make sure we campaign Yes for marriage equality, and to make sure that by the end of the year Australians can finally celebrate coming together knowing that we have achieved something which will truly be a celebration of our national values of fairness and equality."

Mr Cormann said the legislation would apply the same advertising restrictions that are in place for ordinary elections, which prohibit malicious campaigning and bribery.

Ms Marlowe said the LGBTIQ community were a "resilient bunch" who were used to being discussed, but she was worried for the children of rainbow families and vulnerable young people.

"We've been really concerned about how our children will cope with the constant amplification and talk about whether or not their family is a proper family, whether or not their parents are normal, whether we're good parents for them," she said

"We are desperately concerned that a postal plebiscite will unleash that kind of debate and discussion in every school yard, sporting field, and neighbourhood we live in."

"In our own neighbourhood, our friends and neighbours in our street are going to get a ballot in the mailbox, flyers that are not necessarily authorised, banners and posters on the way to school, it's going to be impossible to shield all the children in rainbow families from that kind of ... hate and homophobia."

She advised the community to seek support from mental health and well-being services if they were feeling vulnerable.