A group of conservative Labor MPs are threatening to blow up any last-ditch attempt by Liberal rebels to legalise same-sex marriage, with the numbers in both chambers hanging by a thread.

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.