Prime Minister Kevin Rudd has mounted a spirited defence of same-sex marriage only days out from Australian elections.

Rudd announced in May that he had reconsidered his opposition to same-sex marriage, and he promised to hold a Parliamentary vote on allowing gay couples to marry within 100 days if his centre-left Labor Party wins the election.

Rudd was questioned about his new stance at a nationally broadcast forum in his hometown of Brisbane late Monday by New Hope Church Pastor Matt Prater. Pratt quoted from the Bible a definition of marriage as a man leaving his father and mother to be united to his wife.

Rudd replied that human and social conditions change.

"Well mate, if I was going to have that view, the Bible also says that slavery is a natural condition," Rudd said.

"Because St. Paul said in the New Testament: Slaves be obedient to your masters. And therefore we should have all fought for the Confederacy in the U.S. Civil War," he added.

Rudd said the fundamental principle of the New Testament was universal love.

"If we get obsessed with a particular definition of that through a form of sexuality, then I think we are missing the centrality of what the Gospel […is all about]," Rudd said.

"If you think homosexuality is an unnatural condition, then frankly I cannot agree with you based on any element of the science," he added.

Gay marriage is a point of difference between Rudd and opposition leader Tony Abbott, whose conservative coalition is favourite to win Saturday's elections after six years out of power.

Abbott remains staunchly opposed to same-sex marriage, despite his lesbian sister Christine Forster endorsing marriage equality.

He said Tuesday that Australians should not expect a vote on gay marriage in the next three-year term of Parliament after the weekend election.

"Everyone knows where I stand on this," said Abbott, a former Roman Catholic seminarian. "There are some of my colleagues who have a different position on this."

Australian Marriage Equality national director Rodney Croome described Rudd's defence of marriage equality as "a historic moment in the campaign for the reform."

Croome urged people of faith who support marriage equality to follow the example set by Rudd, who is a Catholic.

"It is unprecedented in Australian history for a prime minister to give such a strong defence of equality for gay people, and it was made all the more compelling because it was from a Christian perspective," Croome said in a statement.

"Mr. Rudd gave a moral and spiritual urgency to marriage equality that will resonate with many Australians, including many Australians Christians, and which marks a historic moment in the debate," he added.

The Labor Party removed its opposition to same-sex marriage in 2011. But the opposition remains opposed and its lawmakers are bound to vote down any legislation that would allow it.

Two bills that would have allowed gay marriage were rejected by Parliament last year.