Religious leaders and coalition MPs are mounting pressure on Prime Minister Tony Abbott to resist calls to allow same-sex marriage.

The latest salvo was fired by 38 religious leaders representing major traditions who have written to the prime minister expressing "grave concerns" about a renewed push to change the Marriage Act.

The signatories include the Catholic and Anglican archbishops of Sydney, a bishop of the Lutheran Church, bishops from various Eastern and Orthodox churches, Christian pastors representing major Protestant denominations, senior rabbis from the Jewish community and Sunni and Shi'ite Islamic leaders.

It comes as Nationals MP Darren Chester broke ranks on Tuesday to announce his support for same-sex marriage.

The parliamentary secretary told ABC TV that the changing mood within his electorate had prompted him to change his mind.

He said he supported a conscience vote on the matter and if one was awarded, he would vote yes.

"I tend to believe there is a certain sense of inevitability about the change, that it will occur at some stage in the future," he said.

There are calls within the coalition for a wider range of options - including civil unions and a national plebiscite - to be considered by the parliament and not just a change to the Marriage Act.

But Australian Marriage Equality national director Rodney Croome said the French system of civil unions - advocated by Social Services Minister Scott Morrison - was ill-conceived.

"In France, legal marriages are performed in civil ceremonies at town halls with an optional religious ceremony that has no legal standing if that's what the couple wants," Mr Croome said.

"We find it appalling that defenders of traditional marriage would prefer to wreck the institution than allow same-sex couples to marry."

A plebiscite would delay the change, he said, and showed that opponents are panicking over the issue.

A Labor private member's bill to change the Marriage Act is due to be briefly debated in parliament next Monday.

Mr Morrison told 2GB radio on Tuesday that those advocating gay marriage should "take a breather" and consider other options such as civil unions.

"I think there's a range of options which haven't been ventilated," Mr Morrison said.

Some coalition MPs believe a change of policy by Mr Abbott could lead to an internal backlash against the leader.

Liberal National Party MP George Christensen likened the disquiet over gay marriage to the 2009 rift when Malcolm Turnbull lost the Liberal leadership over his support for an emissions trading scheme, saying the party membership will also not like being ignored on their opposition to gay marriage.

Parliamentary secretary Steve Ciobo said talk of a split is "hyperventilation".

Nationals deputy leader Barnaby Joyce said he does not believe marriage should be redefined.

Labor frontbencher Brendan O'Connor said Mr Abbott should allow the issue to be resolved and give Liberal MPs a free vote - something they were denied when the issue last arose in 2012.