Prime Minister Kevin Rudd has mounted a spirited defence of his support for gay marriage only days out from Australian elections.

Rudd, a Roman Catholic who regularly attends church, announced in May that he had reconsidered his opposition to same-sex marriage. He promised a bill to create gay marriage in Australia would be voted on in Parliament within 100 days if his centre-left Labor Party wins the elections.

Rudd was questioned about his new stance at a nationally broadcast forum in his hometown of Brisbane 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 "the Bible also says that slavery is a natural condition".

- AP