AUSTRALIA'S political leaders have clashed in a fiery debate which focused heavily on their competing tax plans, as Prime Minister Scott Morrison took the fight up to Opposition Bill Shorten, getting in his face and pointing his finger.

It was a dramatic moment in which Bill Shorten labelled the Prime Minister "a classic space invader", as the two men faced off in the Sky News/The Courier-Mail People's Forum last night.

They each took shots at each other during the forum even as they called for civility to return to political debate.

Mr Shorten narrowly won the debate 43-41, with 16 of the 100 audience members still undecided.

For the first time Mr Shorten revealed Labor would release its policy costings next Thursday or Friday, saying it would be the first Opposition to release them in advance of the final days of the campaign.

The Labor leader tried to upstage the Prime Minister while he was explaining how much tax relief would go to high earners under his plan to flatten tax rates.

As Mr Morrison spoke, Mr Shorten wrote $77 billion on a piece of paper and held it up, referring to a figure by the left-leaning Australia Institute.

"Seventy-seven billion to the top 3 per cent of workers. Nice if you can get it," Mr Shorten said.

It goaded Mr Morrison into his finger-pointing moment, which some likened to Labor leader Mark Latham's aggressive handshake with prime minister John Howard in 2004.

"I wouldn't trust your maths in a heartbeat, Bill," Mr Morrison said.

Mr Morrison targeted Labor's plan to scrap tax refunds of franking credits, saying it would be a $54 billion hit on retirees.

"They're going to tax retirees 27 times more than multinationals," he said.

"You think it's a gift? I think self-funded retirees have worked pretty hard myself."

Mr Morrison took a swipe at Mr Shorten for defending his tax plans, saying, "He's got a lot more taxes to explain."

But the Opposition Leader said it was a lie, saying it was not a tax but closing a loophole.

"There's no rational reason to give someone an income-tax cut when they haven't paid income," he said. 

"This money doesn't grow on trees."

Mr Shorten vowed to deliver Budget surpluses every year and insisted they would be larger than those forecast by the government.

In a fiery end, Mr Shorten heckled Mr Morrison as he was defending his back-down on Malcolm Turnbull's national energy guarantee.

"Peter Dutton didn't like it. Tony Abbott didn't like it," Mr Shorten interjected.

After Mr Morrison warned Labor's plan to increase emissions reduction targets would cost jobs, Mr Shorten said firms were already acting.

The Labor leader said the Coalition was failing the next generation.

"It's selling out the future. It's making us the laughing stock of the world," he said.

Mr Shorten pushed a case for change, saying the economy was not working for average Australians and that there was chaos in Canberra.

"You vote for Mr Morrison, you might get Clive Palmer, who knows," he said.

But Mr Morrison warned that Labor's $387 billion in taxes would hurt the economy when could not afford it.

Labor promised $14 billion more for schools, saying it would pay for more teachers, music and language lessons and school camps.

But Mr Morrison said it was just as important how the money was spent.

"Spending money on schools is obviously important. But what we're spending money on is what's more important," he said.

"Give me a great teacher for my kids any day over a school hall."

There were quieter moments of bipartisanship when the leaders were asked about sexual violence as well as suicide rates among returned servicemen and women.

Both leaders said they were committed to freedom of religion and civility of debate.

But when challenged by a woman in the audience over the ability of people such as Israel Folau to express their beliefs, Mr Shorten turned the question back on her, saying: "What is it you feel you can't say now?"

Mr Morrison said Christians faced discrimination and said he wanted a Religious Discrimination Act.