Bill Shorten tells Christian Lobby he supports same-sex marriage

"I believe in God and I believe in marriage equality," he told the ACL national conference in Canberra.

"I'm a Christian and a supporter of marriage equality under the law."

Religion should never be used as an instrument of division or exclusion, he said.

"I believe our current law does exclude some individuals it says to them that your relationship is not equally valued by the state, that your love is less equal under the law.

"We currently have a law that discriminates against adult couples on the basis of who they love."

Mr Shorten read from the scriptures and said he couldn't remain silent about those who said marriage equality was the first step on the road to polygamy and bigamy and bestiality.

The ACL has campaigned strongly to prevent the passing of marriage equality laws.

While delegates at the conference disagreed with what Mr Shorten had to say about marriage equality, they welcomed the Opposition Leader's openness about the subject.

In a question and answer session after Mr Shorten's speech, ACL managing director Lyle Shelton immediately raised "the elephant in the room" and said "we really appreciate the way you've been fearless and frank with us".

"Obviously there is a point of difference in our views," Mr Shelton said.

Mr Shelton then asked if there could be a civil debate in Australia that "openly canvasses" the pros and cons of changing the definition of marriage.

He said the Hyatt Hotel, the venue for the conference, had been "bombarded with bile and bitter hatred towards ACL" in recent days in an attempt to shut the conference down.

"Yes we should have civility," Mr Shorten said.

"Some people disagreed with me coming here to say my views.

"We're members of Parliament.

"We've got to talk to people."

Mr Shorten received applause when he told the conference "I don't disrespect anyone who holds a contrary view on this question to me".

Lobby group Australian Marriage Equality described Mr Shorten's address as powerful.

"It is the first time a leader of either major political party has used the forum to challenge the agenda of the conservative lobby group," acting director Ivan Hinton-Teoh said.

"It is powerful moment in our national history that a political leader, an affirmed Christian, has had both the confidence and conviction to use this forum to reflect the true diversity of Christian views."

Liberal Democrats senator David Leyonhjelm is working on a new bill to allow same-sex marriage, but won't introduce it until he's reasonably confident the numbers are there to pass it.

The Liberal Party is yet to decide whether to allow a conscience vote on the matter, but Labor MPs have a free vote.

Other speakers at the conference include Liberal frontbencher Michaelia Cash, Labor frontbencher Shayne Neumann and lawyer Roger Kiska from the conservative Christian non-profit body Alliance Defending Freedom.

AAP