Bendigo-based Bridget McKenzie has directly contradicted a fellow Nationals senator by declaring she will vote against a bill to legalise gay marriage regardless of the results of a plebiscite on the issue.

"As a backbench senator, I can exercise my conscience on every single vote that comes before the Senate and I do," Ms McKenzie told reporters at Parliament House today.

"I vote with my conscience on every issue and my conscience on this matter is that I would vote against same-sex marriage."

Ms McKenzie was speaking alongside her colleague Michael McCormack to spruik the Coalition’s regional development policy to be crafted this year.

Her comment’s came directly after Mr McCormack denied the Coalition was split on the issue of gay marriage. 

"If the plebiscite said yes to same-sex marriage, I would support the fact that the will of the people said that," Mr McCormack said.

"I'm not so sure that the Government is split on it.

"We've heard from one or two members who want to stand firm on their own beliefs but I don't believe that the Parliament should go against the will of the people."

Related: McKenzie's brother hits out at marriage stance

Ms McKenzie has been a longstanding opponent of attempts to legalise gay marriage.

In July last year, her brother publicly called her to account for that stance.

In a 370-word letter, Alastair McKenzie, of Melbourne, described growing up gay in the country in the 1990s as "horrific" and labelled his sisters comments as "extremely hurtful".

'Bizarre and extraordinary': Coalition tensions flare over same-sex marriage

Prime Minister Malcolm Turnbull has committed to a national vote on whether to legalise same-sex marriage after the next election, which is expected to cost about $160 million.

His predecessor, Tony Abbott, proposed a plebiscite last year after a bitter debate over a conscience vote inflamed tensions within the Coalition.

When asked whether he would bind his MPs to vote "yes" if that was the plebiscite result, Mr Turnbull told Parliament last year that "the consequence of a 'yes' vote in the plebiscite will be that same-sex marriage will be legal in Australia".

"When the Australian people make their decision, that decision will stick," he said in October. "It will be decisive. It will be respected by this government and by this Parliament and this nation."

But conservative senators are warning they may vote against allowing gays and lesbians to marry even if the Australian public backs the change at a national plebiscite.

Senator Eric Abetz said he would not necessarily vote with the majority while fellow Liberal senator Cory Bernardi went further by saying he would definitely not vote in favour of same-sex marriage, regardless of the public's verdict. 

Senator Bernardi told Fairfax Media on Wednesday: "Even if the public voted for [same-sex marriage], I wouldn't vote for it.

"It goes against what I believe in. This is a substantial issue and, in the annals of public policy, you want to be on the record about your views."

Liberal National MP Warren Entsch, a leading advocate of same-sex marriage, slammed the senators' stance as "bizarre" and "extraordinary". 

"Given he was part of the decision-making process [on a plebiscite], I find it rather extraordinary.

"It makes you wonder why we would spend millions of dollars on a plebiscite if you're not going to respect the result. I find it rather bizarre.

"If people make a decision either way we should respect that. "It will be a very brave individual - either in the House of Representatives or the Senate - who seeks to challenge the views of the Australian people." 

With the Sydney Morning Herald