Labor and the Greens say plans to hold a plebiscite on same-sex marriage are “shambolic” and should be abandoned in favour of a parliamentary vote.

The attorney general, George Brandis, told Sky News on Sunday the government’s mooted plebiscite on the issue would be held shortly after the 2016 election and before the end of the year. If successful, legislation to allow same-sex couples to marry would soon follow, he said.

But the prime minister’s office refused to back Brandis’s timetable, saying the plebiscite would occur “as soon as possible after an election”.

The shadow attorney general, Mark Dreyfus, said on Monday government members needed to “get their stories straight”.



“But most of all, Mr Turnbull should be dropping this wasteful and divisive plebiscite,” he told ABC radio. “Let parliamentarians do the job we are meant to do, which is pass laws, to legislate.”

A Labor government would put a bill before parliament to legislate for marriage equality within 100 days of the election, he said.

The Greens MP Adam Bandt said the proposal was becoming “shambolic”, particularly with some Coalition MPs and senators indicating they would not necessarily vote in line with the outcome of the $160m public poll.

“It’s not even clear that the government knows how to bring it into effect legally,” he told ABC radio.

The education minister, Simon Birmingham, said on Monday the vote should take place at the “earliest possible timeframe after the election”.

“We want to make sure we give the Australian people their say on this issue so that the end result has validity, has confidence for the Australian people, gives them that confidence that the changes, if they are made to the marriage laws, has the support of all Australians,” he told ABC radio.

Birmingham said the government’s intention was to hold an election in September but would keep “all options open” to navigate a “dysfunctional” senate.

Scott Morrison told 2GB radio the details of the vote, including the timing and phrasing of the question, “will go through the normal cabinet process”.

One potential hold-out, the Tasmanian senator Eric Abetz, said the particulars of the plebiscite should be decided by the parliamentary caucus.

“These are matters that the party room still has to sort out in some detail,” he told the Australian.

“If we were to have a plebiscite before the end of the year, and you were to reverse-engineer that, it would make interesting speculation about the timing of an election.”

Abetz said in January he would need to see whether a plebiscite was “above board or whether the question is stacked” before deciding to heed any result in favour of marriage equality.