Deputy Labor leader Tanya Plibersek has branded as "absurd" a government attempt to compromise on its proposed same-sex marriage plebiscite.
Labor MPs are meeting in Canberra on Tuesday to decide whether they will back an enabling bill for a national vote in February.
Ahead of the caucus meeting and a debate in parliament's lower house Attorney-General George Brandis released draft amendments to the Marriage Act late on Monday night.
The changes include redefining marriage as between "two people" instead of "a man and a woman" and allowing ministers of religion to refuse to solemnise a marriage.
Non-religious celebrants can also refuse to marry a same-sex couple on a "conscientious" basis under the proposed legislation.
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Opposition Leader Bill Shorten has been talking down Labor support for the plebiscite, arguing it is a divisive waste of taxpayers' money, and in any case some coalition MPs have indicated they won't accept the result of the national vote.
He believes the best way to achieve marriage equality is to have a free vote in parliament this year.
If Labor does not support the bill it will almost certainly fail in the Senate, with the Greens and key crossbenchers announcing their opposition.
Cabinet minister Simon Birmingham threw Labor a lifeline on Monday, suggesting the planned $15 million funding for the "yes" and "no" campaigns was up for negotiation.
"lf that is something the Labor party want to take out in return for support for a plebiscite they should say so and put it on the table," he told Sky News.
"It would be a reasonable request and one we could consider ourselves."
But Ms Plibersek labelled the offer insincere.
"I think this is an absurd last-minute effort from the government," she told ABC News 24.
"(They) have made it very plain that they are not prepared to compromise on the most fundamental aspects of this plebiscite, including things like the question."
Public funding was only a very small part of the cost of running the "wasteful" exercise.
The Nationals and conservative Liberals have insisted there be public funding.
Nats MP's threat over gay conscience vote
A Nationals MP is threatening to withdraw support for the Turnbull government should it allow a free vote in parliament on same-sex marriage instead of a national vote.
Andrew Broad says a plebiscite is the only way to achieve marriage equality in this term of parliament, because it's what the coalition promised before the July election.
"My support for the government is conditional that we honour our election commitments," he told reporters in Canberra on Tuesday.
Mr Broad refused to reveal whether any others would follow him in withdrawing support, but noted there were many MPs who believe the coalition should honour its election commitments.
Liberal Andrew Laming said a win in the plebiscite would represent a "flying colours victory" for the vote in parliament, hitting back at critics who warn it would be non-binding.
He criticised Qantas CEO Alan Joyce for saying he'd rather wait for same-sex marriage through a parliamentary vote.
"Alan, with the greatest of respect, there's plenty of same-sex Australians not prepared to wait," he said.
Ahead of a Labor caucus meeting, cabinet minister Mathias Cormann urged Opposition Leader Bill Shorten to get out of the way and back the plebiscite.
"Why is he so scared of the Australian people?," he asked.
