Senate powerbroker Nick Xenophon has delivered a potentially fatal blow to the Turnbull government's plan to hold a public vote on same-sex marriage.
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The South Australian independent and his newly elected team of senators announced on Monday the party would not support a plebiscite, days after the Greens said they would vote to block the vote in the Senate.
"This is a matter that the Parliament can and should decide on as a free vote of all members and senators," the Xenophon MPs said in a statement.
"In our representative democracy we are paid to make decisions on behalf of Australians who have voted us into office. This is a decision the Parliament should make now."
Senator Xenophon, new lower house MP Rebekha Sharkie and senators Skye Kakoschke-Moore and Stirling Griff said the planned February plebiscite would not bind MPs and cost at least $160 million.
"We believe this money could be better spent," the statement said.
"Each of us supports marriage equality and we are ready to vote accordingly."
Labor is expected to oppose the plebiscite but is yet to finalise its position.
Opposition Leader Bill Shorten told Labor's shadow cabinet on Monday the plebiscite campaign would subject gay and lesbian Australians to public vilification.
Ridiculing the government's policy, he said Australians faced fines for not voting in the plebiscite while Coalition MPs had pledged to abstain in a subsequent vote in Parliament.
Mr Shorten said the quickest path to marriage reforms was a free vote, committing Labor to pushing for that outcome.
A total of 38 votes are required to block legislation in the Senate, meaning the plebiscite can now only go ahead with support from Labor's 26 senators.
Victorian senator Derryn Hinch is also expected to oppose the plebiscite bill, while Pauline Hanson's One Nation group of four senators would support it.
On Monday, Education Minister Simon Birmingham said that the Coalition would not revert to a free vote if the enabling legislation was voted down.
A leading moderate within the Coalition, Senator Birmingham said a plebiscite was the "only way" for change to happen.
"A message that I would give to the Labor Party, to the Greens, to Senator Xenophon, to anybody who says they support change to allow same-sex marriage is that a plebiscite is the only way they will see that change occur over the next three years," he told Sky News.
Liberal MP Warren Entsch, a longtime advocate for reform, last week vowed not to cross the floor to support a Labor bill if the plebiscite is blocked.
A Fairfax-Ipsos poll in July found seven in 10 voters backed a change to the Marriage Act to allow same-sex couples to wed, while 69 per supported a plebiscite over a parliamentary vote.