Kevin Rudd faced down a Christian pastor who questioned him on his support for gay marriage on Q&A last night, saying his "Christian conscience" led him to back same-sex unions.

The pastor in the Brisbane audience asked the Prime Minister how he could support gay marriage "if you call yourself a Christian".

The Prime Minister responded by suggesting that line of thinking would also make slavery an acceptable proposition.

The pastor asked Mr Rudd: "Jesus said a man shall leave his father and mother and be married ... Kevin, if you call yourself a Christian, why don't you believe the words of Jesus in the Bible?"

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

Mr Rudd's response received loud appaluse from the audience.

Key points Improving welfare payments for single parents a top priority

Improving welfare payments for single parents a top priority Defended support for marriage equality, saying "years of reflection in good Christian conscience" had changed his view

Defended support for marriage equality, saying "years of reflection in good Christian conscience" had changed his view Declined to say whether he will stay on as Labor leader if he loses the election

Declined to say whether he will stay on as Labor leader if he loses the election Admitted he failed to adequately explain why Labor shelved ETS in 2010

"Saint Paul said in the New Testament, 'slaves be obedient to your masters'," the Prime Minister continued.

"And therefore we should have fought for the Confederacy in the US Civil War. I mean for goodness sake, the human condition and social conditions change.

"What is the fundamental principle of the New Testament? It is one of universal love. Loving your fellow man.

"If we get obsessed of the definition of that through a form of sexuality, then I think we are missing the centrality of what the gospel - whether you call it a personal gospel, a social gospel or a spiritual gospel - is all about."

Mr Rudd has pledged to take the first steps towards legalising same-sex marriage if he wins another term.

He said his decision to back gay marriage was made after "years of reflection in good Christian conscience".

"I do not believe people when they are born choose their sexuality. They are gay if they are born gay," he said.

"You don't decide at some later stage in life to be one thing or the other. It is how people are built. Therefore, the idea this is somehow an abnormal condition is just wrong. I don't get that. I think it is a completely ill-founded view."

Welfare for single parents a 'top priority'

During the special television appearance in his hometown, Mr Rudd also vowed to make improving welfare payments for single parents a top priority if he is returned to The Lodge.

Single parents June Connell and Tara McKenna told him they are struggling to provide for their children since being moved from single parent payments to the lower Newstart allowance.

Sorry, this video has expired Kevin Rudd flags social welfare as top priority

The change - which affect parents when their youngest child turns eight - sparked community concern and anger within Labor ranks when it was announced, and Mr Rudd admitted it does not sit easy with him.

"If we are returned, it will be the first thing we address. I mean it. As soon as the budget opens up that amount of space to deal with this challenge, we should act," he told the crowd.

"I was very uncomfortable with it then. I am very uncomfortable with where we are now.

"I am being up-front about it. As soon as the budget allows us a bit of space to act on that, we will."

The Government had previously announced an extra $300 million to help people on several types of welfare payments, including Newstart.

The funding will allow recipients to earn an extra $38 more a fortnight before their payments are reduced.

Renewed calls for Abbott to detail costings

Mr Rudd also defended the Government's use of Treasury and Finance documents to claim the Coalition has a $10 billion hole in its budget savings.

The Treasury and Finance departments have publicly said they had not costed the Opposition's policies, instead costing policy options as provided by Labor before the September 7 ballot was called.

Mr Rudd said if Mr Abbott thinks the claims are wrong, he should release his costings.

"Everything would be clear and on the table as of today if Mr Abbott put it out there in black and white for the entire nation. What are your costings? What are your cuts?" he said.

"If we are going to have a serious debate in this country about where Mr Abbott's $70 billion worth of cuts would fall, Mr Hockey's figure, Mr Robb's figure, he can satisfy that by putting it into the public domain.

"Mr Abbott, because he believes he has this election won, will simply go through to election eve, not tell everyone where the cuts will fall because he is frightened the good folk out there then won't vote for him. It is as cynical as that.

"We will keep the pressure up. People here in this room and across the country deserve to know whether the axe will fall on them."

Earlier on Monday night Opposition Leader Tony Abbott told 7.30 he would release his costings in the coming days, and asked voters to trust that they are as "responsible and scrupulous" as he believes.

During the special Q&A show, Mr Rudd also acknowledged that he failed to adequately explain why Labor shelved its emissions trading scheme in 2010.

When asked whether he had made any mistakes since becoming Prime Minister for the second time, he replied: "It's been a bit too short a time to make another mistake, I think."

"Anyone who pretends in political life that you can somehow go out there and wave a magic wand and get everything right - it's just wrong, you're deluding yourself and deluding the Australian people," he said.

Rudd: 'I entrust my future to the Australian people'

Mr Rudd declined to say if he would step down as Labor leader if the Coalition wins the election.

"I'm not going to engage in hypothetical-land. I'm not in that frame of mind," he said.

"I am fighting this election campaign as the Prime Minister of the country and the leader of the country and the leader of the Labor Party."

Mr Rudd was asked how history would remember his leadership should the Coalition claim victory on September 7 - as outcome polls suggest is likely.

Sorry, this video has expired Rudd proud of achievements on economy and jobs ( ABC News )

"How about we get a question which ends with 'if you do win the election'," Mr Rudd joked, adding he is proud of how he helped steer Australia through the worst of the GFC.

"What I have sought to do as Prime Minister of this country so far is to be as energetic and passionate as possible about carving out a vision for Australia's future.

"What is stunning is when you meet people out there in the normal world, and not this political bubble in which we live, who go through challenges in their life which are infinitely harder than I've ever had to deal with.

"My dear departed mum said if you think you are having a hard time, Kev, there is always someone who has 10 times as many problems.

"I take great strength and encouragement from the wonderful folks in the country I speak to every day. I entrust my future to their judgment."

Q&A says Mr Abbott was invited to take part in the show, and also offered his own solo appearance, but is yet to take the offer up.