Labour and National have always differed over ways to protect the environment but their policies have never been as far apart as they are now.
It has become what politicians call a litmus issue, a defining test of difference which starkly separates the main parties.
The Greens are aligned with Labour, while ACT is National's staunch supporter.
A bill going through parliament has been the catalyst for the policy contest that will run through to the 2014 election.
The Climate Change Response (Emissions Trading and Other Matters) Amendment Bill changes the emissions trading scheme (ETS).
Labour had an ETS in the pipeline before it lost the 2008 election, National rewrote it and passed it in 2009.
The rewrite was not as strong as the original and the bill now being debated further weakens the ETS.
Basically, the ETS brings industries under its regime and they have to pay for their greenhouse gas emissions through buying carbon credits, or selling them if they cut their emissions.
The problem has always been when they start paying and how much, issues which are called transitional measures.
Some sectors are already under it but others waiting to enter the ETS are having their deadlines extended. Agriculture, which produces more than half New Zealand's emissions, is being given an indefinite reprieve.
The government's case is that other countries aren't exactly cracking along with their emissions reduction programmes and New Zealand can't afford to impose costs on its own industries which would damage their international competitiveness.
It also argues that most of those costs inevitably filter down to households which are still feeling the effects of the recession.
"This bill will ensure that the emissions trading scheme effectively supports the government's economic growth priorities by keeping carbon costs at a level that is appropriate in the light of the current economic climate," says Climate Change Minister Tim Groser.
That is the core of the matter.
Labour and the Greens accuse the government of sacrificing the environment on the alter of economic growth and argue the consequences will be catastrophic.
"I think the minister has forgotten why we are doing all this," says Labour's Moana Mackey.
"Frankly, if this is the kind of legislation that we are going to be passing we might as well get rid of it, because we don't have an emissions trading scheme."
The Greens are beside themselves.
"It subordinates climate change to economic growth," says Kennedy Graham.
"It implies that economic growth is the principal criterion of the quality of life."
Graham says the government is approaching climate change as if reducing greenhouse gas emissions is a matter of choice for the human race.
"These are not matters of choice," he says.
""Emissions reduction is an ecological imperative. Nature does not do deals, other than with itself, and certainly not with humanity."
Following Graham in this debate was ACT leader John Banks, whose party doesn't think there should be an ETS at all.
"In a perfect world we wouldn't be wasting the time of parliament changing an act that should never have existed," Banks says.
His opinion of Graham's speech was: "I have never heard such claptrap in this parliament... a bogeyman tirade, humbug."
Banks says he always knows the government is on the right track when there is "a howling and a wailing from Labour and the Greens".
He is going to hear a lot more of that.
The bill will be enacted because the government has got the numbers - just.
Banks and United Future's Peter Dunne will give it 61 votes against 60 mustered by Labour, the Maori Party, the Greens, NZ First and Mana.
Climate change legislation, fundamental to New Zealand's interests, should have cross-party consensus allowing it to survive a change of government without fundamental change.
That isn't going to happen.