The Greens will "never" form a coalition with National while Simon Bridges leads the party.
Green Party co-leader James Shaw told Newshub Nation on Saturday morning there's a very simple reason.
"I would never empower someone with as little personal integrity as Simon Bridges to become Prime Minister," he told host Simon Shepherd, saying switching sides is "absolutely" off the table.
"We are making huge progress as part of this Government, and we want this Government to be returned."
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It's a question that comes up every election cycle - could the Greens do a deal with the centre-right, if it means getting into Government?
Former leader Russel Norman in 2017 said it would never happen, even under a co-leader like Shaw, who's seen as relatively business-friendly compared to past party leaders.
"If they did so there would be very significant internal division, which would cause significant damage," he told The AM Show. "They would lose the votes of many, many people who voted for them at the last election. It's kind of a suicide note really, for the Green Party."
Shaw at the time kept the door open for National, saying it was the "polite" thing to do.
"I have said that I will hear the Prime Minister out if he wants to make his case," said Shaw, who took over from Dr Norman in 2015.
"I'm keen for the Greens to have a chance at Government, because we've never really had that full opportunity before."
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National deputy Paula Bennett said she was keen to get the Greens on board, but "I don't think they are up for it".
Shaw's declaration on Saturday comes after the Greens launched an attack ad targeting Bridges. The clip, which featured a comedian putting on an exaggerated New Zealand accent, angered many of the party's supporters.
"It was an attempt at humour," co-leader Marama Davidson - appearing alongside Shaw - told Newshub Nation. "We wanted to highlight the dangerous inaction of Simon Bridges on climate change. It was not at all supposed to be an aggressive attack. It didn't fly, it didn't land, and we took it down."
Bridges has regularly defended his environmental record, saying under his leadership the party would do better than in the past.
"National emphasises the environment, and under me I think we'll emphasise it a little more, because we understand that it's important to New Zealanders and it's important to me," he said during his first appearance on Newshub Nation as leader last year.