The Turnbull government's proposed plebiscite on same-sex marriage has formally been killed off by the Senate, where it was defeated 29-33 in a late-night vote on Monday, amid a warning the decision would delay marriage equality "for years".

Months of speculation and political posturing culminated in Labor, the Greens, the Nick Xenophon Team and Derryn Hinch combining to defeat the proposal, which would have seen same-sex marriage decided by the Australian people in February.

The decision - an inevitability since Labor pledged to oppose the plebiscite four weeks ago - will force a new conversation about marriage equality that will divide the Coalition and threatens to destabilise the Turnbull government.

Conservatives will resist any attempt to shift away from the plebiscite policy, demanding no action on same-sex marriage until at least the next election. Most observers expect any change on that front would blow up Prime Minister Malcolm Turnbull's leadership.