Divisions have emerged within the federal government over how the question of same-sex marriage should be put to the Australian people.
Attorney-General George Brandis on Thursday dismissed cabinet colleague Scott Morrison's suggestion that a constitutional referendum could settle the issue, labelling the proposal "entirely unnecessary".
The attorney instead agreed with his Labor counterpart Mark Dreyfus, with both insisting the High Court in 2013 had already made it clear that marriage was a matter for the commonwealth parliament to legislate.
"That is an unambiguous, a unanimous and a recent statement by the High Court that the marriage power as currently written is ample to provide the parliament with the power to legislate for same-sex marriage," Senator Brandis said.
"There is no doubt whatsoever that a referendum to amend the constitution is not necessary."
Mr Dreyfus dismissed Mr Morrison's suggestion as "nonsense" saying a referendum could cost about $120 million.
Mr Morrison hit back on Twitter saying unlike a plebiscite, a referendum was binding and compulsory.
"Oz people should decide this not politicians, judges or lawyers," he tweeted.
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Fellow cabinet minister Malcolm Turnbull said amending the constitution would not resolve the matter because any change still required legislation to pass parliament.
He believes any plebiscite on same-sex marriage should be held before the next federal election so it doesn't distract from other important issues.
Prime Minister Tony Abbott won't say whether he and other opponents of same-sex marriage will abide by any popular vote that supports a change to existing laws.
He said the government would have more to say before the next election about whether it intends to hold a plebiscite or a referendum.
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Meanwhile, a growing number of Liberal MPs including Wyatt Roy, Dean Smith and Teresa Gambaro have indicated they will cross the floor if it comes to a vote in parliament.
Senior Labor frontbencher Penny Wong warned giving Australians the say on same-sex marriage was a green light for hate speech.
"A plebiscite is not about the Australian people, Tony (Abbott)'s plebiscite is all about the leadership of the Liberal Party," she told reporters in Canberra.
Fellow Labor senator Sam Dastyari described the concept as a "rort".
"Frankly we need to call it for what it is and that's bullshit," he said.
The Australian Christian Lobby says the matter should be decided by a vote of the people.
Australian Marriage Equality would rather a free vote in parliament but have thrown their support behind a push from the Greens and crossbench senators to require a plebiscite on same-sex marriage be held at the next federal election.
A cross-party bill currently being drafted, which so far has the support of the Greens and senators Ricky Muir, Jacquie Lambie, Nick Xenophon, Glenn Lazarus and David Leyonhjelm, would require Australians to cast a vote on gay marriage at the same time they go to the polls for the election.
It would also require the plebiscite question come from parliament rather than the prime minister.
