The Labor MPs have warned there has been no discussion within their party about how to deal with a private members' bill by Liberal senator Dean Smith, including on the need to suspend standing orders.

It threatens to derail any back-door attempt to make same-sex marriage law, as has been contemplated by a handful of moderate Liberal MPs.

Two Labor senators, Jacinta Collins from Victoria and Helen Polley from Tasmania, said there were procedural issues to work through and it should not be assumed that all Labor MPs would agree to debate a private members' bill.

"I wouldn't make that presumption," Senator Collins said.

The senators told Fairfax Media that Labor would ordinarily form a working group to sort out procedural issues related to matters of conscience.

"We still have a conscience vote," Senator Polley said. There were "procedural things we'd have to work through, and there has been no discussion recently about how that might follow", she said.

Both senators noted they had not seen Senator Smith's proposed bill or discussed it with their colleagues.

"It depends what the legislation says," Senator Polley said. "There needs to be protection of small businesses who need to be protected, as well as those from a faith base."

Up to six Labor senators are opposed to same-sex marriage and could derail the bill in the Senate. In part, that is why same-sex marriage advocates argue any ambush should begin in the House of Representatives.

But there are also concerns about a handful of lower house Labor MPs who have not expressed support for marriage equality.

Those include Chris Hayes, the chief opposition Whip, Maria Vamvakinou and Anthony Byrne, who on Thursday confirmed to Buzzfeed News he would vote "no" in a conscience vote.

Even if they can be relied on to help suspend standing orders, there is no guarantee they would support the same-sex marriage bill itself. Neither Mr Hayes or Ms Vamvakinou responded to requests for comment.

Senior Labor sources are confident that if the numbers are close, hold-outs within the party will be convinced to vote "Yes" or at least abstain to avoid embarrassing Opposition Leader Bill Shorten.

But same-sex marriage advocates inside and outside Parliament are aware of how close the numbers are and remain concerned that if the matter gets to a vote in coming weeks, they may fall short.

Other Labor senators known to oppose same-sex marriage are Don Farrell, Alex Gallacher, Chris Ketter and Deborah O'Neill, most of whom have ties to the socially conservative Shoppies union.

Senator Farrell has said he would vote for marriage equality from 2019, when it will become mandatory for Labor MPs. He and other senators were contacted for comment.

Labor starts with 26 senators. If six abstain, it brings the total number of votes down to 70, with a majority of 36 required. Labor's 20 would combine with nine Greens (presuming Larissa Waters and Scott Ludlam are replaced), the Xenophon trio, Derryn Hinch, David Leyonhjelm and Senator Smith to reach 35.

The numbers are complicated further because Labor same-sex marriage supporter Gavin Marshall leaves for a secondment at the United Nations in mid-September.

One Nation has a conscience vote. Leader Pauline Hanson opposes gay marriage, and has said she would vote for it only if a plebiscite showed that was what Australians wanted. Jacqui Lambie has articulated a similar position.

Fairfax Media understands Senator Smith hopes to find a second Liberal senator prepared to cross the floor, with some advocates suggesting Jane Hume or James Paterson as possible candidates. However, Senator Hume and Senator Paterson told Fairfax Media they would not cross the floor on same-sex marriage.

On Thursday, Prime Minister Malcolm Turnbull called an emergency meeting of the federal parliamentary Liberal Party to debate same-sex marriage on Monday afternoon.

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