Fairfax Media revealed at the weekend that a secret push was under way whereby Coalition MPs and senators would be allowed to reflect the views of their electorates or states, instead of the national result.

Prime Minister Malcolm Turnbull also said the proposed plebiscite - which based on previous polls would be expected to pass comfortably - would not bind his MPs or even his cabinet ministers.

Mr Turnbull has said he hoped to hold a plebiscite before the end of 2016, if re-elected on Saturday.

Nationals MP David Gillespie, an opponent of marriage equality, told said he would vote in line with his conservative-leaning north coast NSW seat of Lyne, even if the majority of Australians vote "yes".

"I've got a very conservative electorate," he said. "I'll respect the views of my electorate."

Liberal senator for the ACT Zed Seselja confirmed he would probably abstain from voting if the plebiscite was carried, despite his electors being very likely to support same-sex marriage.

"I would reserve the right to abstain. That would be a very likely scenario in those circumstances," he told ABC radio on Tuesday.

Conservative Liberal MPs Andrew Nikolic in Tasmania and Andrew Hastie in Western Australia have previously indicated they would throw their support behind the decision of their electorate, rather than the national outcome.

But other MPs in more urban seats have promised to honour the will of the people, despite their personal opposition to marriage equality.

Liberal MP Alex Hawke, who represents Mitchell in Sydney's north-west, said he would vote "no" in the plebiscite but "yes" in the Parliament if it is carried nationally. He also expected most of his colleagues to follow suit.

"I'd campaign for a 'no' vote but I'd support the national view if there was a national result," he said.

"Most people are thinking about the national result; that's what a plebiscite is. Ultimately it has got to be respected."

Liberal MP Craig Kelly said he would vote for same-sex marriage if it was supported by the majority of Australians, despite his personal preference for the status quo.

However, he added a rider that some MPs might need to be given some leeway to vote in accordance with their electorate.

"If you are in a seat that voted overwhelmingly against it, you should be entitled to vote according to your electorate," Mr Kelly said.

In the Victorian seat of Wannon, Minister for Veterans' Affairs Dan Tehan said he would also vote in line with the national result, despite personal opposition to marriage equality.

Attorney-General George Brandis issued a statement denying any "deal" had been reached allowing MPs to vote according to the will of their electorates.

But Mr Turnbull has already said it was Liberal Party "tradition" to allow a conscience vote on such issues, which would give same-sex marriage opponents room to move.

That prompted a cry of outrage from Labor, which argues the $160 million plebiscite is damaging and unnecessary, especially if MPs will not be bound to the national result.

Neither Labor nor the Greens will say whether their senators would support legislation to enable a plebiscite if the Turnbull government is re-elected.

Opposition Leader Bill Shorten said on Tuesday that marriage equality would be his first legislative act if Labor is elected.

The plebiscite was the negotiated outcome of a marathon joint party room meeting under the leadership of Tony Abbott last year. 

On Monday night, Mr Abbott's former chief of staff Peta Credlinpredicted the issue would cause chaos within the Coalition comparable to that of the emissions trading scheme in 2009, which split the party and ended Mr Turnbull's first stint in the leadership.

- with James Massola