National leader Bill English is on the back foot in the polls and with the crowd at a Christchurch debate against Labour's Jacinda Ardern, but tried to rally with passionate defences of National's record on poverty, dirty rivers and social welfare.

The debate began just after the latest 1 News Colmar Brunton poll showed National had slipped down two points over the past week to 39 per cent - four points behind Labour which was steady on 43.

It gives Labour a four point lead over National - and more worryingly shows National's vote starting to slide away. It was its lowest since 2005.

Asked if he was worried by the polls, English said "not really."

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He said as the election drew near there would be more focus on policy and ideas.

"It's going to be about whether we need more taxes or not - we don't - and whether we can reach into the depths of our social problems."

One of the most passionate exchanges was over the issue of dirty rivers - after Ardern said nothing had been done in National's nine years, an angry English said Ardern was insulting farmers who had invested a lot of money in measures such as fencing off waterways and wintering sheds.

Ardern denied she was insulting farmers, but maintained more had to be done to clean up waterways. She said New Zealand had to be able to "trade on a brand we can be proud of."

Ardern also accused English of doing little for child poverty or homelessness and English launched into his defence of his 'social investment' policy saying it was capable of transforming the lives of the most vulnerable.

The crowd jeered when English mentioned his finance spokesman Steven Joyce as one of his lineup of talent - clearly the result of controversy over the so-called $11.7 billion hole.

English said he stood by Joyce and maintained Labour's plan included two or three years of budgets in which there was no new money outside health and education.

He said he had delivered zero Budgets in which there was no extra spending and it was difficult and should only be done in a depression - not when there were surpluses.

As he spoke, Ardern said "take a breath, Bill" and accused him of lying. "It's a matter of trust."

"I've done eight Budgets - I know how it works," English replied.

English also tried to trip Ardern up on what he called her "vague" policies and emphasis on vision, saying detailed actions were needed not just talk of values.

She denied Labour's policy set was 'vague,' saying she could speak in detail on any policy area - the only one in which she could not was around tax. She maintained it was the right decision to leave that to a Tax Working Group in her first term because of the impact it could have on housing affordability.

English had begun by praising Christchurch for its resilience but the audience laughed when he spoke about the rebuild being completed in a few years.

English also reprised his pitch about the uncertainty in Labour's tax policy.

"Now the stardust has settled you're starting to see the policy. You're being asked to vote for a committee which will decide everything after the election."

He said those who would benefit from National's tax package and the families under its social services approach.
"They can't afford to trust it all to a committee."
However, there was further scoffing when he mentioned housing and child poverty.

Ardern began her opening by saying "this stardust won't settle."

She said Labour could do better than National - and could do it without jeopardising the economy.

Adapting the 'hope' and 'yes we can' catchphrases of former US President Barack Obama, Ardern said the election was a choice between "risk and hope."
She set out a list of "cans" such as "we can clean up rivers" and ended with "we can do it together, so let's do it together."

Ardern said the Green Party would still be her first port of call if she is in a position to form a government, despite polls showing she was in sight of being able to form a government with NZ First alone.

Ardern made the call in Stuff debate tonight and when asked why bother, she said "my word's my bond."

Asked what she might offer NZ First leader Winston Peters, she ruled out having anyone other than a Labour minister in the finance role. Asked about Peters, English said he could teach Labour a thing or two about fiscal discipline.