Bill English was on the defensive in a sometimes-fiery leaders' debate tonight after a new poll showed his party would struggle to form a Government after the election.

The Stuff Leaders' Debate in Christchurch coincided with another bad result in the 1News-Colmar Brunton poll, which placed National in the 30s for the first time since 2005.

The Labour Party, on the hand, would have options in forming a government, including a possible coalition with the Greens and the Maori Party.

Labour leader Jacinda Ardern said the Green Party would still be her first port of call after the election, despite the poll showing she was in sight of being able to form a government with NZ First alone.

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Asked at the Stuff Debate why she would bother talking to the Greens, she said: "My word is my bond".

Prime Minister Bill English was on the defensive in the Stuff Leaders' debate in Christchurch tonight, which coincided with another bad poll result for National. Photo / Stuff
Prime Minister Bill English was on the defensive in the Stuff Leaders' debate in Christchurch tonight, which coincided with another bad poll result for National. Photo / Stuff

Ardern also reiterated that NZ First leader Winston Peters would not be Prime Minister or Finance Minister in a government she led. English made the same commitment, but was more complimentary about Peters, saying he could "teach Labour a thing or two about fiscal discipline".

In front of an audience which appeared to favour Labour, English got the biggest jeers of the night when he mentioned his finance spokesman Steven Joyce as one of his main talents - clearly a response to Joyce's recent attacks on Labour's so-called "fiscal hole" in its budget.

This was a different Ardern from the previous, relatively civilised debates. She was fired up, more aggressive, and regularly interjected while English was speaking. That turned into anger when English stood by Joyce's claim about Labour's fiscal hole.

English was "maintaining a lie", she said repeatedly.

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

There were a couple of Barack Obama touches from Ardern. She adapted his "hope" and "yes we can" catchphrases and even his knuckle-point for emphasis.

English again went after Ardern on her "vague" policies, saying she needed to provide more detailed actions rather than just talk of values.

Jacinda Ardern and Bill English clash over several issues during the Stuff Leaders debate in Christchurch

Asked if he was worried about the latest poll, English said "not really". There would be a greater focus on ideas and policy as the election day drew near, he said.

English returned to his main line of attack, the uncertainty about Labour tax policies.

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

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. Ardern denied she was insulting farmers, but maintained more had to be done to clean up waterways.

It was a different JAcinda Ardern from the previous, relatively civilised debates. She was fired up, more aggressive, and regularly interjected while PM Bill English was speaking. Photo / Stuff
It was a different JAcinda Ardern from the previous, relatively civilised debates. She was fired up, more aggressive, and regularly interjected while PM Bill English was speaking. Photo / Stuff

Talking to reporters after the debate, English said the latest poll was not what he was seeing in National's polling.

"But it is helping people sharpen up, whether they really want to take the risk of policy that is going to slow the economy or keep going, whether they want to keep cash in their pocket or have Labour take it out."

The worst news for National is that, realistically, it probably would not be able to form a Government with the combination of parties in this poll scenario.

It could not form a coalition with New Zealand First unless it also included the Greens or the Maori Party, parties which New Zealand First would find difficult to work with.

After the debate Ardern repeated her claim that National had introduced 18 new taxes in the past three terms. She said farmers should not be concerned by a 1c to 2c tax on 1000 litres and after the election she would discuss it further.

"I want to meet with people. I want to work this through."

Commenting on the poll she said she would be working hard to try to keep the lead and put Labour in the strongest position after the election.