Plebiscite bill defeated in the Senate

The federal government's proposed same-sex marriage plebiscite has been defeated in the Senate.

Legislation to enable a national vote was defeated 33 votes to 29, with Labor, Greens, the Nick Xenophon Team and independent Derryn Hinch opposing it.

Liberal backbencher Dean Smith was absent from the chamber, after earlier speaking out against the plebiscite.

Attorney-General George Brandis had previously warned that voting against the plebiscite would result in delaying same sex marriage in Australia for years.

He said Labor's decision to block the plebiscite "one of the more cynical exercises in politics that I have ever seen".

Senator Brandis urged his colleagues to "stop playing politics with gay people's lives" and "get out of the way".

But the opposition argued that a long campaign against marriage equality would lead to a vicious debate that would be harmful to the mental health of members of the LGBTI community.

Labor leader Bill Shorten, who supports marriage equality, said a public campaign against gay marriage could lead vulnerable people to suicide.

"Let me be as blunt as possible: a No campaign would be an emotional torment for gay teenagers and if one child commits suicide over the plebiscite, then that is one too many," Mr Shorten said in parliament in September.

The future of marriage equality in Australia is now uncertain with conservatives in the Coalition believed unlikely to shift their position on achieving same-sex marriage through a plebiscite.

Australian Marriage Equality chairman Alex Greenwich said his organisation would now lobby for the issue to be decided by a parliamentary vote before the next election.

"The past 12 months has been a very difficult time for the LGBTI community," he said.

"We now need to make sure that we channel the frustration of the plebiscite into passionate advocacy to achieve this reform."

The plebiscite was originally proposed by Tony Abbott as prime minister in August 2015.

The move was seen as a strategic balancing act to appease those in the Liberal party who wanted the government to enact a change that had widespread community support and also silence conservatives in the party who were opposed to gay marriage.

Mr Turnbull did not support the policy at the time but adopted it when he ousted Mr Abbott as prime minister in September last year.