Crossbench senators are backing an Australian Greens bill calling for a gay marriage plebiscite before the next election.
Prime Minister Tony Abbott has suggested a plebiscite be held after the election, due in 2016.
The government’s official position is to retain the traditional definition of marriage as between a man and a woman.
Government MPs are not allowed a free vote on any same-sex marriage bill.
The government is also considering holding a referendum, which is an expensive process and usually only done when the nation’s constitution needs to be changed.
Australia’s marriage laws can be changed by parliament.
The new cross-party bill would mean Australians would cast a vote about legalising same sex marriage at the same time as the general election.
The plebiscite, which is non-binding, would act as a gauge of the public’s position on the issue.
Public opinion polls have found a majority of poll respondents approve of same sex marriage.
Greens leader Richard Di Natale accused the government of “spectacular” leadership failure on the matter.
“But if a plebiscite is the only option on the table, it must happen at this election and the parliament must choose the question,” he said.
Independent senators Ricky Muir, Jacqui Lambie, Nick Xenophon, Glenn Lazarus and David Leyonhjelm have backed the Greens’ bill.
Separately, backbench government MP Warren Entsch plans to introduce a cross-party bill next week to legalise same-sex marriage.
Mr Entsch and three other government backbenchers have said they would cross the floor to vote in favour of the bill.
The Labor Opposition has pledged to introduce same-sex marriage legislation within the first 100 days if it wins the election