Prime Minister Malcolm Turnbull said on Sunday he would press ahead with plans to introduce the enabling legislation for the plebiscite, which he expected would pass through the House but conceded "we'll see how it goes in the Senate".

Mr Turnbull said he expected Labor to support the bill and added that if it was true - as Fairfax Media reported on Sunday - the opposition intended to block the plebiscite, gay people would be left waiting for the right to marry their partners.

"The Labor Party must want to delay same-sex marriage for a very long time, if they are briefing that," he told the ABC's Insiders.

"There is no question that the fastest way, the way to guarantee that there is a vote in the Parliament on gay marriage in this Parliament, is to support the plebiscite. Personally, I have no doubt the plebiscite will be carried and the same-sex marriage legislation will then sale through the Parliament."

Government frontbencher Christopher Pyne on Sunday accused Labor of being hyper-partisan "villains" and running the risk of no action on same-sex marriage for three years.

"Bill Shorten and the Labor Party are ... putting at risk the possibility of marriage equality in Australia. They are the villains in the piece," he told Sky News.

"Mr Shorten cannot guarantee that there will be a vote in the Parliament on marriage equality. So he's quite prepared to say to all those same-sex couples who want to marry ... he's quite prepared to tell them 'no', we're not going to allow that to happen.

"So the the only villain in the piece here is Bill Shorten, not the Coalition."

Mr Pyne, a supporter of same-sex marriage, did not categorically rule out the possibility of legislating for marriage equality through a free vote, but reiterated the government's policy was to hold a plebiscite.

He argued that if marriage equality were brought about by a free vote, it would lack "legitimacy" because opponents would argue they never had the opportunity to campaign against it and put the question to the people.

Mr Turnbull conceded there were arguments to make against the plebiscite such as its potential cost - which ranges from a minimum of $160 million to up to $250 million.

"[But] the worst argument, the absolutely worst argument against a plebiscite is to say that it wouldn't be passed. So if Labor is seriously saying that, if they are saying, 'Don't consult the Australian people because they won't give you the answer you want,' it is the most anti-democratic argument," he said.

"Bill Shorten should stand up today and disown that, otherwise he has absolutely destroyed the credibility of his case against the plebiscite."

The Prime Minister inherited the promise to hold a plebiscite from his predecessor, Tony Abbott, and holding one - rather than allowing a conscience vote on the floor of the Parliament, which would almost certainly see gay marriage legalised - is part of the Liberal/National coalition agreement.

Mr Turnbull, a moderate, would face a massive backlash from conservatives within his own party were he to abandon the plebiscite and attempt to hold a free vote in the Parliament, as Labor, the Greens and some crossbenchers have urged.

Conservative MPs who oppose same-sex marriage have already said that if the plebiscite legislation is defeated, they will consider the debate closed.

Labor Senator Sam Dastyari said the party still believed some Liberals could be convinced to cross the floor to support same-sex marriage in a free vote. But he acknowledged it was a "difficult" choice to risk delaying marriage equality for several years.

He said he believed a plebiscite would ultimately be carried by the Australian people but that "some really horrible things will be said" in the process.