Conservative senator says MPs entitled to make up their own minds and vote accordingly even if the $160m plebiscite determines Australians are in favour

The prominent conservative senator Eric Abetz says every Coalition MP will be free to decide how to cast their parliamentary vote on same-sex marriage, contradicting the prime minister and, according to the marriage equality lobby, rendering the promised $160m national plebiscite pointless.

Malcolm Turnbull supports marriage equality and originally argued against Tony Abbott’s “circuit-breaker” policy of a plebiscite after the next federal election, but then promised conservative Liberal colleagues and the Nationals he would keep the policy when he sought the Liberal leadership.

In October Turnbull told parliament his government would abide by the decision made by the Australian people and anyone arguing otherwise was “not living in the real world”.

“When the Australian people make their decision, that decision will stick,” he said. “It will be decisive. It will be respected by this government and by this parliament and this nation.

“But let me tell you this. If you imagine that any government, this government or any government, would spend over $150m consulting every Australian on an issue of this kind and then ignore their decision, then they really are not living in the real world.”

But Abetz, a former minister in Abbott’s government, has argued the people’s decision in a plebiscite would not necessarily be decisive.

“I would need to determine whether [the plebiscite] really is an accurate reflection [of the national view], whether it is all above board or whether the question is stacked, whether all sides received public funding,” he told Guardian Australia.

“But everyone knows my view is very strongly that a marriage between a man and a woman is the foundational institution for socialising the next generation. And every member of parliament will make up his or her mind after the plebiscite is held. People will take into account the views of the electorate, the views of the nation and their own personal views.

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“It would be up to each member to decide whether the plebiscite accurately reflects the views of the Australian people, whether it reflects the views of their electorates and whether it is good or bad public policy in their view.

“There will be people in the parliament who could not support the outcome of a plebiscite whichever way it went. If the plebiscite came back with a ‘no’ vote on marriage equality would [pro marriage equality backbencher] Warren Entsch drop his campaigning on the issue. I think not.”

Australian Marriage Equality’s national director, Rodney Croome, said Abetz’s comments “show clearly that there is no point in having a plebiscite and spending $160m when politicians are going to make up their own minds anyway. With or without a plebiscite, this issue will be resolved by a free vote of all members of the parliament, so let’s just get on and have that vote.

“People are attracted to the idea of a plebiscite because at first glance it seems like a circuit breaker, but what Eric Abetz is saying shows it won’t be conclusive at all, and he’s right about that.”

Abetz’s comments come as Abbott flies to the US to address a far right group, the Alliance Defending Freedom, which funds legal action against same-sex marriage and, according to its website, is at “the forefront of the international legal battle for religious freedom”.

Last year the successful Liberal candidate in the West Australian Canning byelection, Andrew Hastie, also said he would not be bound by a plebiscite but by the way his electorate voted.

“When it comes to politics, I will be guided by the people of Canning,” he said during the campaign.

“For example, on the issue of gay marriage, I personally don’t support it, but I do support Australian men and women deciding it rather than politicians. This means that if the plebiscite comes back and the people of Canning have supported it, I will vote accordingly.”

At the time Entsch said that position made no sense.

“We can’t bind parliamentarians’ votes but the very strongly endorsed view of the party room was that the outcome of a people’s vote should be adhered to. Either you have a people’s vote or you don’t. There would be no point in having a people’s vote and then ignoring it. We really can’t have it both ways.”

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On Tuesday Entsch said the idea that the issue should be resolved by plebiscite had been Abbott’s call, and if he, or other conservatives, wanted to change the decision, the whole discussion would be reopened.

“Tony Abbott is entitled to talk to whomever he likes but we arrived at this position for a plebiscite at his behest,” he said.

“He made that call after a long party room discussion which decided that our position in favour of the Marriage Act as it stands was not applicable after the next election.

“Having a plebiscite after the next election was his call. He decided that this should be determined by the Australian people after the next election. If he, or anyone else, wants to make any changes to that then that would have to be a decision of the party room.”

The attorney general, George Brandis, is drafting legislation to define the question to be put to a plebiscite after the federal election, the exemptions that would apply to churches and other organisations, and whether voting would be compulsory. It will go to cabinet, and the Coalition party room, next month.

The Labor leader, Bill Shorten, said he was concerned Abbott and the conservatives were “pulling the strings” inside the Coalition on the matter.

“What Mr Abbott does is up to him ... What worries me is that his views are commonly held within the right wing of the Liberal party.

“Malcolm Turnbull would like everyone to believe he’s the one person in the Liberal party. The truth of the matter is that many Liberal MPs and the powerbrokers of the Liberal party think like Mr Abbott and they’ve simply installed Malcolm Turnbull as the frontman.

“The real problem is: who’s pulling the strings in the Liberal party? I am greatly concerned that Malcolm Turnbull may be the presentable frontman of the Liberal party, but the people pulling the strings, their views are identical to Tony Abbott’s and that’s not good for Australia’s future.”

Last year Entsch suggested the parliament could vote on same-sex marriage during this term, with the result activated only after a successful post-election plebiscite. At the time Abetz described that idea as a “thought bubble” and an “ambush”.

Some MPs, such as Queensland’s Andrew Laming, have polled their electorates on the issue, but Abetz dismissed that tactic.

“I have been somewhat bemused by the view that politicians might poll their electorates to decide their views,” he said. “People elect us so that we exercise our own best judgments on all the issues that come before us.”