Some former Green MPs say the party will be "gagged" and become "lapdogs" to Labour under the parties' new cooperation agreement.
The deal was signed on Sunday morning after a majority of Green members voted in favour of it.
While it landed co-leaders Marama Davidson and James Shaw with ministerial roles, some previous MPs say they've "sold out" to Labour.
Former Green MP Sue Bradford said the agreement will "really mute" her party's voice.
"They will simply be an ineffectual lapdog of Labour for the next three years."
Former Greens co-leader Russel Norman believes the party would've been stronger in opposition.
"Once they're inside government they're effectively gagged, so they can't set the agenda on many issues but particularly core issues like climate change," he said.
The Greens needed a 75 percent majority of member delegates to say yes for the deal to go ahead - the party got 85 percent.
"What we got yesterday was a super majority so that's a really clear mandate for us," Davidson said.
Shaw added he doesn't feel gagged by the agreement.
"We are really delighted that we do have a win-win agreement here," he said.
But ultimately, Labour holds the super majority and doesn't need this deal to govern.
"When you've got no negotiating power there's no point selling yourself for trinkets, which is what they've done," Bradford said.
Those "trinkets" are ministerial roles for the Greens. Davidson becomes Minister for Prevention of Family and Sexual Violence and the associate Housing Minister. Shaw retains his position as Minister of Climate Change and picks up associate Minister for the Environment. Both Davidson and Shaw will be outside of Cabinet.
The new ministers are both bound by Cabinet rules, which means no speaking out publicly against the government on their portfolio areas, though they can make a quiet note on the Cabinet minutes if they disagree.
Prime Minister Jacinda Ardern said the agreement means the parties "actually don't have to agree".
Despite having the mandate to govern alone, Labour said it wants to work with all parties on an MMP review and political donation laws. A four-year parliamentary term is also on the table, which Ardern said would go to referendum.
"No politician wants to be seen to be feathering their own nest and traditionally it has gone to referendum, and I imagine that would be a most likely scenario," she said.