The three litigants in the suit will be the independent member for Denison, Andrew Wilkie; the national spokesperson for Parents and Friends of Lesbians and Gays, Shelley Argent; and Victorian mum of three and Rainbow Families Victoria convenor, Felicity Marlowe.

They will be represented by Ron Merkel QC. The suit is expected to be filed in the next 24 hours.

The government announced its plans for the voluntary postal vote on Tuesday, saying that ballots for the $122 million vote could start arriving in letterboxes as early as September 12.

The vote, which finance minister Mathias Cormann described as a "survey", will be conducted by the Australian Bureau of Statistics, rather than the Australian Electoral Commission — news that many found surprising.

The government believes it has the constitutional power to fund the vote under a clause in the current Appropriations Act that allows up to $295 million of taxpayer money to be spent on "urgent" and "unforeseen" circumstances.

Long-time marriage equality advocate Rodney Croome said in a press conference on Wednesday that, in Merkel's point of view, there are many points of the government's plan that may be unconstitutional.

BuzzFeed News understands there are three main tenets to the legal challenge: whether the ABS has the authority to collect the data it has been asked to by the government, whether it qualifies as an urgent and unforeseen expense under the Appropriations Act, and a third point on the Commonwealth exceeding its powers.

Wilkie said it was "truly frightening" to see the government try and bypass the parliament in this way.

"From my point of view, it's about governance ... about the parliament exercising its authority," he said.



Marlowe said she had joined the suit because she was "desperately worried" about the impact a plebiscite campaign would have on same-sex parented families.

"There are many, many families out there that are standing here today anxious and worried about the impact in their schoolyards, sporting communities, local neighbourhoods — even just thinking about people letter-boxing up and down their street," she said.