There's been a massive setback for the gay marriage vote in Australia

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A plan to hold a popular vote on whether Australia should allow same-sex marriage has suffered a setback after a political party announced it would not support the proposed public vote.

The Nick Xenophon Team supports marriage equality, but the party says its senators will not support legislation to authorise the vote, which it says would effectively become a £92 million opinion poll without legal weight.

Even if most Australians voted for same-sex marriage, conservative government lawmakers could still block the reform in parliament, the party said.

Labor leader Bill Shorten (Rob Griffith/AP)

Another party that favours gay marriage, the Greens, announced last week that it would also oppose the vote.

That leaves the opposition centre-left Labor Party as the government's only hope of getting the Senate to back a popular vote on same-sex marriage.

Labor leader Bill Shorten supports marriage equality but has recently stepped up his attacks on the plebiscite plan.

"The quickest path to resolving this issue would be a vote in the parliament and that's what we will be seeking to do in the coming days and weeks," he said.

The Nick Xenophon Team and Greens agree with Labor that parliament should decide without waiting for a non-binding popular vote.

"We should never put questions of human rights to an opinion poll," Greens leader Richard Di Natale said.

Prime Minister Malcolm Turnbull agreed to hold the plebiscite in a deal with gay marriage opponents within his party. In return, those opponents backed Turnbull in an internal leadership ballot that toppled prime minister Tony Abbott a year ago.

Australian Prime Minister Malcolm Turnbull speaks at the government offices in Sydney

Turnbull, a gay marriage advocate, had previously spoken out against such a public vote that could create painful divisions in Australian society.

Gay marriage lobbyists are generally opposed to the plebiscite, which they argue was initiated by MPs who hope it fails.

Plebiscites and referendums - legally-binding popular votes - rarely manage to change the status quo in Australia. And some marriage equality advocates warn that a lost plebiscite would probably set back their cause for decades.