The same-sex marriage plebiscite is dead in the water and marriage equality stuck in limbo after Labor ended months of speculation and confirmed it will block the proposal in the Senate.

Opposition Leader Bill Shorten said he could "not in good conscience" support a public vote on same-sex marriage, because it was expensive, non-binding and would inflict untold harm on the gay and lesbian community, especially children.

Talk has now turned to what comes next, with Prime Minister Malcolm Turnbull at this stage declining to rule out the free vote he once supported, and Coalition conservatives making it clear they will not truck any shift away from the "plebiscite or nothing" approach.

Attorney-General George Brandis, who late Monday night unveiled the government's proposed amendments to the Marriage Act if a plebiscite were successful, said Labor had "driven a stake through the heart of marriage equality in Australia". In Question Time, Mr Turnbull accused Labor of "wringing every ounce of political gain out of this debate".