The parties to the challenge, which could be lodged as early as Wednesday, include Tasmanian independent federal MP Andrew Wilkie and marriage equality advocates Shelley Argent and Felicity Marlowe.
The Turnbull government has proposed the Australian Bureau of Statistics run the postal vote, which will start in mid-September unless a court injuncts it.
However, long-term same-sex rights advocate Rodney Croome said legal advice provided by barrister Ron Merkel QC found there were constitutional problems with the ABS running the poll.
"Mr Merkel feels that the idea of a postal vote running through the ABS may exceed the ABS's authority, particularly when we consider whether a postal vote on marriage equality is statistic gathering or not," Mr Croome said.
It comes after the Senate has blocked a government move to restore the same-sex marriage plebiscite to its agenda following an emotional debate.
Labor, the Greens and the Nick Xenophon Team on Wednesday had the numbers to tie a vote at 31-all, meaning the move was defeated.
Liberal backbencher Dean Smith, whose attempt to initiate a parliamentary vote on gay marriage was stymied by his party colleagues this week, supported the government but chose to sit with the crossbench.
Emotions ran high during the debate with Labor senate leader Penny Wong urging the government not to expose her children with her female partner to hate.
"We love our children and I object, as does every person who cares about children... to being told our children are a stolen generation," Senator Wong told parliament in an impassioned speech, referring to comments made by the Australian Christian Lobby.
"It's exposing our children to that kind of hatred."
The government was trying to restore the defeated legislation rather than putting it back to the lower house because it wasn't confident its own members would support it, Senator Wong said.
The government will now proceed with a $122 million postal vote which does not require legislation.
A postal vote would see ballot papers in mailboxes from September 12 and a result declared on November 15.
There would be no publicly funded "yes" and "no" campaigns.
Senator Wong described the option as an expensive and damaging stunt.
The money could pay for three million GP visits, or thousands of teachers.
Instead it would be spent on a useless opinion survey given that conservative Liberal senators, such as Eric Abetz, have indicated they will not change their vote regardless of the outcome, she said.
"They simply cannot countenance people like me and others being equal," she said.
Indigenous Labor senator Malarndirri McCarthy told the Senate the government had "wimped out" and not given thought to the language barriers Indigenous people will face.
"In the Northern Territory, we have 43 town camps 72 remote Indigenous communities and 500 homelands," she said.
"What consideration have you given to over 100 Aboriginal languages and translations that should take place?"
Acting Special Minister of State Mathias Cormann argued a public vote would be a "unifying moment for the country", enabling those on the losing side of the argument to more readily accept the result.
He rejected claims the debate around a plebiscite would be damaging, insisting the government trusted the Australian people to have a respectful debate.
