The same-sex marriage plebiscite is dead in the water after Labor MPs finally agreed to block the proposal in the Senate at a caucus meeting on Tuesday.

The decision casts marriage equality off into the never-never, with the Turnbull government adamant a plebiscite is the only way of achieving reform in this term of Parliament, and one Coalition MP already threatening to bring down the government if that is changed. 

Labor tussled with the issue for weeks but hardened its resolve after consulting with gay and lesbian advocacy groups and mental health experts, with Opposition Leader Bill Shorten at one point declaring a public vote on same-sex marriage could lead vulnerable people to suicide.

MPs voted unanimously to oppose the plebiscite on Tuesday, with only Mr Shorten, shadow attorney-general Mark Dreyfus and marriage equality spokeswoman Terri Butler speaking on the matter. It was resolved in less than 15 minutes.

Mr Shorten later said he had not heard a good argument for the plebiscite and reiterated Labor's view that it was unnecessary, expensive and had the potential to cause mental ill health among LGBTI people.

"It is a waste of money. It will not bind Liberal MPs. It will cause potential harm to gay people and families and Australians just want us to get on with it," he said.

"Children do not need to go to school in the climate of a plebiscite and have the integrity of their parents' relationship challenged.

"I could not in good conscience recommend to the Labor Party that we support the plebiscite ... the evidence became overwhelming of the harm it would cause."

Asked directly if there was anything the Turnbull government could do to salvage the plebiscite and make it more palatable to Labor, Mr Shorten reiterated that he had not heard a good argument for spending $200 million on a non-binding vote that would harm people's mental health.

Conservatives in the government will celebrate the result, which comes fourteen months after the plebiscite was first conceived at a marathon Coalition party room meeting under former prime minister Tony Abbott.

Even with Labor unified in opposition to the plebiscite, the vote in the Senate will still be close. On the expected numbers, it would be defeated 37 to 35, with senators Cory Bernardi and Lisa Singh overseas, Labor senator Stephen Conroy having resigned and Liberal senator Dean Smith abstaining.

Australian Marriage Equality co-chair Alex Greenwich said the vote would be "very tight" and called on Labor to ensure all its senators voted in accordance with the policy adopted at caucus.

"The political reality is the plebiscite is now dead," he said. He expressed optimism that a free vote could be achieved in this term of Parliament despite the vehement opposition of the Coalition's right wing, insisting the process of agitating for a free vote "can only begin once the plebiscite is finally dead".

Mr Shorten also reflected that position, saying Prime Minister Malcolm Turnbull was "capable of changing his mind" as he had done on this issue previously, as well as several other policy areas.

"When you put to me that Malcolm Turnbull has a rock solid position, I just see that as the start of the debate," the Labor leader said.

It came as Nationals MP Andrew Broad threatened to bring down the government if it switched to a free vote on same-sex marriage, indicating his support for the government was "conditional" on it honouring all its election promises.

"The government that I am a part of and will remain a part of is conditional on the fact that the only way that there will be a change to the Marriage Act in this Parliament is a plebiscite," he said. "My position as a member of the government is very clear on that."

But there is recognition even among Coalition conservatives that the issue will not disappear from the radar once the plebiscite is defeated in the Senate, but urged the party to stay with position it took to the election.

"It's not going to go away," one conservative Liberal MP told Fairfax Media on Monday. "I think we will put ourselves in a very precarious position if we break faith on this policy."

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