It may not yet be a battle of equals. But one thing is clear from yesterday's first head-to-head clash in Parliament between John Key and Helen Clark. There is now going to be a real battle.

You could almost see National MPs' hearts swelling with pride as they watched their new leader cut loose and confront the Prime Minister with a ferocity rarely seen from Don Brash.

When putting his questions, Dr Brash would stand upright, often with one hand in his pocket, and read directly from his notes, losing much impact in the process. In contrast, Mr Key yesterday ended up dispensing with his notes, instead eyeballing Helen Clark directly as he spoke.

Mr Key cuts a slight figure amongst some of the more ample physiques and lanky frames on National's front bench. But this was a telling moment. Enough for his colleagues to be confident that normal business has resumed after the three-year failed experiment with Dr Brash, who never adapted to Parliament's rough and tumble.

Instead of worrying whether the leader will make it through the afternoon without mishap, National MPs can now entertain the possibility that he might make life really uncomfortable for Helen Clark.

The emphasis is still on the "might".

Yesterday's exchange was too brief to draw any firm conclusions about that - and the two leaders are not scheduled to square off again before Parliament rises for Christmas.

Their encounter also contained more than the usual element of theatre. Both knew they were on display and were accordingly well-rehearsed.

Mr Key had cleverly chosen to quiz the Prime Minister on her stated aim of making New Zealand "carbon neutral" - that for every bit of carbon dioxide that ends up as greenhouse gas, an equivalent amount is extracted from the atmosphere and absorbed or buried.

She has yet to detail how we might attain this extremely ambitious objective given the country's greenhouse gas emissions keep rising while the planting of carbon dioxide-absorbing forests is declining.

That line of questioning also showed Mr Key making good his promise to take National's fight to Labour on issues which Labour regards as its preserve.

He ended up getting more from the exchange than he expected but probably not what he wanted: an offer of multi-party talks on climate change.

It is debatable whether Labour or National - both out to "own" climate change - want the issue shuffled down the dead-end street of multi-party discussions.

However, Helen Clark's offer was an equally clever response to Mr Key's questions, turning the tables on his efforts to undermine her.

He had begun by asking her whether she had sought Ministry for the Environment advice before going public with the idea of carbon neutrality.

The response was predictably dismissive. "No. I'm pleased to say that I am not utterly dependent on others for what I think."

When he asked for the date when New Zealand would become carbon neutral, she ducked the question, saying "a lot earlier than those still in climate change denial like the member".

It was at this point Mr Key threw away his notes and suggested, that when it came to this PM, "the country shouldn't listen to her rhetoric, just look at her record".

As the cheering on the National benches abated, Helen Clark responded in kind to this attack on her credibility. When it came to taking people at their word, she replied acidly, she would take Mr Key at his when he told Parliament last year he was not sure climate change was even a problem. "What's changed Mr Key?" she barked.

The universal verdict was an honourable draw. Mr Key could be well pleased. But he also got a timely reminder that like the previous four National leaders she has faced, Helen Clark will show no mercy in endeavouring to see him off as well.