A High Court challenge to the federal government's proposed postal vote on same-sex marriage will be lodged this week.
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.
There were also questions around whether the government can pay for the postal ballot without the parliament passing legislation.
Ms Argent, the spokeswoman for the group Parents and Friends of Lesbians and Gays, said the non-compulsory postal vote was "designed to fail" by former prime minister Tony Abbott.
"What they could do is have the free vote (in parliament) - there is more dignity and respect," she said.
"I am, as a mother, standing up and that is why we are going to the High Court."
Mr Wilkie, who was due to meet with Mr Turnbull on Wednesday afternoon, said it was "frightening" to watch the government exceed its powers and bypass the parliament.
"It is not okay for the government to think it is a law unto itself ... and somehow authorise the expenditure of more than $100 million of taxpayers' funds."
Mr Croome said he would soon commission research to gauge whether those seeking marriage equality wanted advocates to join the campaign for the "yes" case, or boycott the postal ballot.
A survey last year of 5500 people found 78 per cent of voters wanted the focus of advocates to be on stopping a plebiscite in its tracks, compared with 16 per cent who wanted work on a "yes" campaign and six per cent who were undecided.