The Green Party says a Zero Carbon Act introduced in the first term of Government is its bottom line for working with Labour.
Leader James Shaw made the announcement at the minor parties' leaders' debate hosted by Three's The Nation.
Mr Shaw asked to pick which of his babies - eliminating poverty, cleaning up rivers or climate change - he'd choose if he could only pick one.
"In the first 100 days, the first thing I would do would be pass a Zero Carbon Act that would make it legally binding that New Zealand would be net zero carbon economy by 2050," he said.
He said it was a bottom line for working with Labour.
"It would oblige future governments to put in place policies that are consistent with that goal."
The Green Party wants to scrap the Emissions Trading Scheme, which requires polluters to buy permits in order to emit CO2 and other gases.
The ACT Party agrees, and both say they want a carbon tax which requires all polluters to pay.
Labour wants to keep the scheme, but bring farmers into it, while National wants to keep the ETS, but rely on science advances to reduce emissions.
Labour leader Jacinda Ardern wouldn't make any promises on a carbon tax, but assured Green leaners they'll get the same result with Labour.
"We've already committed to legislating our targets in law. That's a commitment we've outlined a while ago now," she told Newshub.
The gaps for both parties appear when it comes to when agriculture will be charged for polluting the air.
Ms Ardern says she'll leave that to her climate commission, while Mr Shaw is releasing policy closer to the election.