Labour leader Jacinda Ardern has threatened Australia with retaliation if it goes ahead with plans to make New Zealanders pay more for tertiary education, saying "if they lock us out of tertiary education, we will lock them out of it here".

It was one of several surprising pledges from Ardern and National leader Bill English in the second leaders' debate on Newshub last night.

English also got in a surprise pledge under questioning from moderator Patrick Gower, saying he would commit to a target of getting 100,000 children out of poverty.

That came after he launched into a strong defence of National's record on child poverty, pointing to measures such as increasing the benefit rate and its Families Incomes package in this year's Budget.

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"Compared to five years ago, 60,000 fewer kids wake up tomorrow morning in a benefit-dependent household. That's a big change ...I'm proud of what we have done and I want to really have a got at it."

Ardern called for a legislated target to reduce child poverty, saying her entire reason for being in politics was to eliminate child poverty. She pointed to Labour's own families package as delivering more to those families.

Jacinda Ardern speaks during the Newshub Decision 2017 Leaders' Debate in Auckland. Photo / Michael Bradley / Newshub
Jacinda Ardern speaks during the Newshub Decision 2017 Leaders' Debate in Auckland. Photo / Michael Bradley / Newshub

Ardern also repeated the pledge of former Prime Minister John Key to resign rather than increase the age of superannuation.

Labour campaigned to lift the super age to 67 in the last two elections, but is staying with 65 this election. Ardern said she believed starting contributions to the Super Fund was enough.

It was an echo of Key's pledge in 2008 which was aimed at putting to bed speculation National would increase the super age - and helped it get into government. But it prompted a jab by English: "Well isn't that letting down her generation, because they are going to have to pay the bill for that."

It was a reference to Ardern's answer to an earlier question about what she had that English did not - "generational change and a vision for the future of New Zealand".

English is proposing lifting the super age to 67 from 2034.

Bill English and Jacinda Ardern both fired up in the second debate. Photo / Newshub
Bill English and Jacinda Ardern both fired up in the second debate. Photo / Newshub

It was the second time the two leaders had met and both were a lot more fired up and willing to confront each other, getting jabs in on issues from immigration to infrastructure.

Immigration was another area of difference and Ardern defended Labour's plan to cut immigration by 20,000 - 30,000 a year, saying it was aimed at immigrants such as those on "low value courses" rather than those in skills shortage areas.

Gower asked Ardern how the 100,000 houses in its Kiwibuild policy would be delivered if it was cutting back on immigration and could not get the builders to build them, prompting English to quip Labour would give visas to New Zealand builders to work in Auckland.

On immigration and the pressure it was putting on Auckland's infrastructure, English said that was being fixed as things such as the Waterview Tunnel had shown.

The tunnel got some applause but Ardern scoffed, saying "it's not a plan or a vision, it's a tunnel".

That prompted a return fire from English about Ardern's values and vision: "Let's get rid of all the orange cones and see if people can live on the vision."

Patrick Gower hosted the second leaders' debate. Photo / Newshub
Patrick Gower hosted the second leaders' debate. Photo / Newshub

English said the government was now "flat out" building hospitals and roads - but Ardern said National's plan had been "selling houses to each other and immigration".

Gower also tackled the leaders on housing policies, and English defended his record, saying government actions had led to a flattening of prices as supply started to kick up and loan to value ratios took effect.

Ardern said Labour did not want to see people lose the value in their homes, but it should not be as hard as it was for first home buyers to get into the market.

"I don't accept out teachers, nurses and police officers can't get into their first homes. You've had nine years, it's time to hand over to someone with a vision and a plan."

English said Labour's policies such as a possible capital gains tax and clamping down on immigration would stall housing supply.

English said the capital gains tax was inevitable for Labour to afford its promises, and that would affect the same police, teachers and nurses Ardern claimed to be trying to get into their first homes.

Jacinda Ardern. Photo / Newshub
Jacinda Ardern. Photo / Newshub

Talking to media after the debate, Ardern said Labour's pledge to eventually provide three years of free post-school study or training would apply to Australian citizens living here.

However, if Australia did make university significantly more expensive for New Zealanders living there, the free policy would not apply to Australians here.

"I would match that with Australians being treated as international students here as well. I just think that is just fairness.

"There's no need [to retaliate] right now."

Ardern was not long leader of Labour when Australian Foreign Minister Julie Bishop said she would find it hard to trust a Labour Government, after Labour MP Chris Hipkins asked citizenship questions following contact with an Australian Labor staffer and during questions over the citizenship of Australia's Deputy Prime Minister Barnaby Joyce.

Tonight, she said the relationship with Australia would remain strong under a Government led by her.

"Absolutely and I think they would expect that if they made a change that affected New Zealanders that we would then have a change here that would affect Australian students. It doesn't affect the importance of our overall relationship."

Ardern was asked about abortion law during the debate and said it should be removed from the Crimes Act.

She told media she would put forward legislation to remove it from the Crimes Act but voting on that would be a conscience matter for MPs.

"But it is my view."

Bill English. Photo / Newshub
Bill English. Photo / Newshub

Speaking after the debate, English said Ardern's statement saying she would retaliate was playing to a view New Zealand could have a big influence on Australia.

"We are the small one, they are the big one. We have hundreds of thousands more New Zealanders in Australia than they have Australians here and the way we've got results so far has been to work constructively with them, patiently making the case."

English said he had committed to the target of a reduction of 100,000 in child poverty during the debate to drive home the message that National was concerned about it and committed to it.

English's firm commitment to a target of reducing child poverty by 100,000 - a higher figure than Labour's - was one of the surprises of the debate.

The Childrens' Commissioner and other advocates have long called for a target but National had refused. English said in an election when the public were listening closely, it was a "great opportunity" to set out the target.

"We have been going in this direction for some time ... it's certainly been a bit frustrating that people believe that because we are a National Government we are not interested in that when we have invested an enormous amount of time in these families who need support and better incomes.

Tonight was an opportunity to make it really clear to people that part of what we want to achieve as a re-elected government is a significant reductions in child poverty."

English said the definition he was working on for poverty was the OECD measure of 50 per cent of the median income - which included about 150,000 children.

He said National's families package in the Budget - a mix of tax cuts, and increases to Working for Families and the Accommodation Supplement - was expected to reduce that by 50,000.

National believed it could offer a further similar-sized package in two or three years' time - which would help reach the 100,000 target.

He said the social investment measures he had spearheaded to identify those at risk of welfare dependency and a life of crime and provide early support to help prevent it was also important.

"So tonight I really just crystallised it to make it clear to the public the track we are on."

He said he felt the debate had gone well and he had been able to highlight National's plans on housing and the economy compared to Labour's "vague" mix.

He was also standing by National's claims earlier in the day about a hole in Labour's fiscal plan.

Ardern said she wasn't clear about what measure English was using when he pledged during the debate to cut the number of children in poverty by 100,000.

She said she approached English after the debate to ask what measure he planned to use but he wouldn't tell her.

She said she had been pushing National to set a target for nine years without success.

"I'm pleased. But I am still surprised as well."

- Additional reporting Nicholas Jones