Senator Carr's comments come after former prime minister Kevin Rudd said on Monday he had changed his mind on gay marriage and now believed parliament should make it legal.

"I think any observer would say there's an underlying trend as members of parliament give it more consideration and listen to the arguments to legalising same-sex marriage," Senator Carr told ABC radio on Tuesday.

He also acknowledged there was "considerable support" for legalising same-sex marriage among coalition MPs.

Mr Rudd said any legal recognition for gay marriage should make religious institutions exempt from having to change their own practices.

Last September Mr Rudd was one of 98 MPs who voted against a marriage equality bill.

But on Monday he recounted a recent conversation with a gay, "God-botherer" former political staffer who told him he hoped to get married to another man one day.

The encounter led Mr Rudd to rethink his position.

Senior Liberal George Brandis said Mr Rudd's comments were more about personal ambitions than about the issue of same-sex marriage.

"It's not about same-sex marriage," he told Sky News.

"What it tells you is that Kevin Rudd has not given up, Kevin Rudd is at it again."

Senator Brandis said the former prime minister was continuing to "dance this dance" in front of the media for as long as he possibly could because his caucus supporters had not given up hope of him returning as Labour leader.

"Every time they see a depressing opinion poll result, they're at it again," he said.

The Australian Christian Lobby says Mr Rudd's announcement was a huge disappointment for Christians.

This left their hopes for the "preservation of marriage" with the coalition and Christian-based minor parties, managing director Lyle Shelton said.

"If this is an attempt to wedge Julia Gillard, it will cost Mr Rudd the last of his following in the Christian constituency," he said in a statement.

Mr Shelton said Mr Rudd's position on legalising same-sex marriage would mean parents would have their children "taught the mechanics of homosexual sex" in school sex education classes.

Labour backbencher Stephen Jones said Mr Rudd had followed a path taken by many MPs who had begun instinctively with scepticism or opposition.

"They (who) sat down and thought about it, or who have been confronted with personal stories, they realised it is time to change the law," Mr Jones told ABC radio.

Mr Jones said sentiment on the issue was changing and legalising gay marriage was "inevitable".

"At some stage over the next five years, we will have a change of law in our country," he said.

Mr Jones introduced the same-sex marriage bill that was defeated in parliament last year.