Labor now holds the key to whether a national vote on same-sex marriage goes ahead.
The Nick Xenophon Team on Monday confirmed its opposition to a bill to enable the plebiscite, in the wake of a plan by the Greens to vote it down when it comes to parliament.
One Nation's four senators say the plebiscite should go ahead, but not until the next election to reduce the cost to taxpayers.
With the coalition government needing nine extra votes in the Senate to get its bill passed, its only hope lies in convincing Labor.
The Labor caucus has not yet seen the legislation, but leader Bill Shorten told colleagues on Monday the most effective way to deliver marriage equality was for the parliament to vote directly on a change to the Marriage Act.
Same-Sex marriage activists march in the street during a Same-Sex Marriage rally in Sydney (AAP) Source: AAP
"That's what we will be seeking to do in coming days and weeks," he told a meeting of his shadow ministry in Canberra on Monday, ahead of the 45th parliament opening on Tuesday.
Senator Xenophon and his three colleagues say legalising same-sex marriage should be decided by a free vote in parliament before the next election.
If a private member's bill passed the lower house - with Liberal MPs crossing the floor to vote with their consciences - it would also pass the Senate, he said.
"We are elected by the voters to make decisions. We are paid very well to make those decisions," Senator Xenophon told reporters in Canberra.
Independent senator Derryn Hinch said in a statement on Monday he would vote against the plebiscite bill.
A better and quicker route would be a conscience vote in the parliament, he said.
However, Liberal Democrats Senator David Leyonhjelm, who previously brought in his own bill to allow same-sex marriage, urged his fellow crossbenchers to reconsider their position and support the plebiscite.
"If it's left to the Greens, Saudi Arabia will have marriage equality before we do," he told AAP on Monday.
Cabinet minister Simon Birmingham warned if the plebiscite was blocked, there wouldn't be a same-sex marriage vote in parliament.
Government backbencher Warren Entsch, a long-time advocate for same-sex marriage, says if the plebiscite bill is blocked that should be the end of the matter until the next election.
Sharyn Faulkner, from Parents and Friends of Lesbians and Gays, said Labor should now join with the crossbenchers to veto the plebiscite.
"We are grateful that more and more politicians are seeing a plebiscite for what it really is - a platform for hate and a delaying tactic," she said.
