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She blasted National's agricultural policy this afternoon, calling it "backwards-looking," and "hugely disappointing".
"The policy that has been put out by the National Party would take New Zealand back decades," she told reporters in Christchurch this afternoon.
Greens co-leader James Shaw's disdain over the policy was also clear when he said National was "anti-science and anti-climate".
"National is once again showing contempt for future generations in its proposal to dismantle huge progress on the environment and climate action."
The thrust of their frustration stems from two elements of National's plan:
• To remove the review process around introducing agriculture into the Emissions Trading Scheme (ETS) in 2022
• To pass seven changes to the Zero Carbon Bill, including a review of the methane target
The latter point is the one Ardern appears to be taking the strongest issue with.
Under the leadership of Simon Bridges, National voted with the Government to pass the Zero Carbon Bill.
It is understood getting the National caucus onside to vote for the bill caused tension within the party.
Although National did vote for the bill, it signalled at the time it would be making some changes to the legislation if it won the election - including the changes announced in today's policy.
But Ardern said today that there had appeared to be a consensus when it came to climate change within National.
"It is hugely disappointing to see that change in position from them."
"To now see the National Party change its position is deeply disappointing – it takes New Zealand backwards and I think it also hurts our brand."
National's policy does not suggest the party would repeal the Zero Carbon Act.
Rather, a number of changes including reviewing both the methane target and the use of the level of forestry offset to achieve climate-change targets.
Ardern was also critical of National's plans to remove the 2022 review process, when it comes to potentially moving farmers into the ETS.
Last year the Government decided that instead of putting agriculture into the ETS – meaning farmers would pay for their carbon emissions – it would give them a few years to bring their emission down.
If it could do that, the Government would look at an alternative way to price on-farm emissions starting in 2025.
But if not, agriculture would be put into the ETS.
National's plan takes away this stick and promises to work with farmers to further develop the He Waka Eke Noa plan – a scheme where Government and the industry work together to bring down emissions.