The coalition party room on Tuesday will consider cabinet's decision to hold a plebiscite on February 11 and give $7.5 million each to the two sides of the debate.
MPs are also expected to endorse the question that will be put to voters: "Do you support a change in the law to allow same-sex couples to marry?"
But a compulsory vote and funding for the plebiscite needs parliament's approval, something Labor's Graham Perrett thinks won't happen.
"All this kowtowing to the extreme right of (the coalition) will all come to nought and be a total waste," he told ABC radio.
"It's got a snowflake's chance in hell of getting through the Senate."
Marriage equality advocates says public funding for yes and no cases makes the plebiscite even more unacceptable.
"We cannot countenance taxpayers' money being spent on what is likely to be hurtful, harmful and even hateful campaign materials from the 'no' case," Parents and Friends of Lesbians and Gays spokeswoman Sharyn Faulkner said.
Greens senator Janet Rice is worried the gay and lesbian community now faced a summer of "hateful, hurtful, homophobic" speech.
