Liberal frontbencher Christopher Pyne asked Nationals leader Warren Truss not to bring his party to the marathon Coalition same-sex marriage debate on Tuesday.

The ABC has confirmed the call was made after Mr Pyne had earlier accused Prime Minister Tony Abbott of "branch stacking" for making the debate a matter for the joint party room and not the Liberals alone.

Mr Pyne made a passionate plea for a free vote at the morning Liberal Party meeting.

He argued that including the more conservative Nationals would skew the numbers towards those arguing to toe the existing party line of binding the Coalition to supporting traditional marriage.

When news of what he said leaked it outraged National MPs and senators.

Queensland LNP backbencher George Christensen warned that attempts to exclude the Nationals would backfire on the Government.

"If you know we've got ministers that are saying they don't want the Nationals in there when decisions made, then boy oh boy isn't it going to be tricky for them when the Nationals take a different point of view on every government policy," he said.

"So it was only right and appropriate and fair that the Nationals were in that room."

Brandis declares referendum 'entirely unnecessary'

Meanwhile, Attorney-General George Brandis has declared a referendum on same-sex marriage is "entirely unnecessary" despite Social Services Minister Scott Morrison calling for one.

After the Coalition party room voted on Tuesday night to maintain the policy of opposing legalising gay marriage, Mr Abbott suggested a referendum or a plebiscite on the issue to allow the Australian people to decide.

A referendum is a vote to change the constitution, while a plebiscite measures public opinion, through a popular vote, which the government can choose to act on.

On Thursday Mr Abbott would not specify which he preferred.

"Precisely what form that putting it to the people might take is something that we will have more to say about between now and polling day," he said.

Mr Morrison was pushing for a referendum.

"The type of issue that could be canvassed under Section 51 of the constitution — simply at the moment, in Clause 21, it just says 'marriage'," he said on the ABC's 7.30 program on Wednesday night.

"You could equally put in there 'opposite and same-sex marriage' and clarify very clearly what the meaning of the constitution is on this question, and to reflect [what] some would argue has been a societal change since the constitution was first written."

Senator Brandis dismissed Mr Morrison's suggestion, pointing to a High Court decision two years ago that found the Parliament had the power to legislate on marriage, in relation to the Commonwealth overriding same-sex marriage laws in the ACT.

"That is an unambiguous, an unanimous, and a recent statement by the High Court that the marriage power as currently written is ample to provide the Parliament with the power to legislate for same sex-marriage should it choose to do so," Senator Brandis told Sky News.

"So no constitutional referendum is necessary in this case," he said.

"The way you test public opinion on vexed social issues or important social issues is by plebiscite."

Public vote a 'rort, joke', Dastyari says

Labor senator Sam Dastyari described the Government's moves to hold a public vote on same-sex marriage after the next election as "bullshit".

Labor argues that given the difficulty of getting the double majority of state and national support required for constitutional change, the proposal is designed to fail.

Some conservatives and Christian groups that are opposed to gay marriage believe a failed referendum could push the issue off the national agenda for many years.

A plebiscite needs to reach a lower bar, only requiring national support, but the Government would not be not compelled to act on the result.

Speaking to reporters outside Parliament House on Thursday morning, Senator Dastyari said the Government's push for a public vote was designed to stop gay marriage being legalised.

"What we've seen happen on the issue of marriage equality in the past couple of days is disgraceful," he said.

"It's a rort, it's a joke, it's a false premise and frankly we need to call it for what it is and that is bullshit.

"The Australian public have had their say, they want these laws passed, we can pass them now, frankly we should be passing them now."

Government MP Warren Entsch has been given approval for a private members bill to legalise gay marriage, but he has acknowledged the outcome of the party room vote this week suggests there would not be enough support in the Lower House even if Coalition MPs had a conscience vote.

Two Coalition backbenchers, WA senator Dean Smith and Queensland MP Wyatt Roy, confirmed they were prepared to cross the floor on the issue.