"For too long, we have set targets and not done what's needed to achieve them," Ardern said.
"That's why Labour will set up an independent Climate Commission to examine what reductions can practically be achieved by each sector and recommend emissions reductions targets."
Since becoming leader last month, Ardern has placed greater emphasis on environmental policies, including climate change.
She reiterated today that climate change is "my generation's nuclear-free moment".
She said that the Emissions Trading Scheme would remain at the core of Labour's plan to hit its emissions targets.
Agriculture's inclusion in the scheme would be gradual and farmers would initially get a 90 per cent discount on their carbon credits.
That would help bring emissions down "without hurting agriculture's contribution to the economy", she said.
Labour also wants the public sector to lead the way on reducing emissions, and it would require state-owned enterprises and other government departments to pursue low-carbon options and technologies.
Under the policy, the public sector could only buy electric cars for their fleet unless there were "exceptional reasons" not to do so.
Labour's new goal will require a significant reversal in emissions. New Zealand's net emissions have risen 64 per cent since 1990.
National has set a target of reducing emissions to 11 per cent below 1990 levels - a goal which is locked into the Paris Agreement on climate change.
National aims to reach the target by buying carbon credits, reducing transport emissions and investing in research to reduce agricultural emissions. It opposes putting agriculture into the ETS.