The Green Party will turn its back on a long-held party position and "swallow a dead rat" for the "stability of Government".
New Zealand First negotiated the 'waka jumping' bill into its coalition agreement with Labour. It's a policy long opposed by Greens.
New Zealand First's changes will give parties the power to expel members of Parliament. Disgruntled MPs can currently break off from the party and "jump waka" to another party or set up another party.
The Greens believe such legislation gives parties the power to force MPs to toe the party line. The party opposed similar legislation in 2001 and 2005.
Green MP Eugenie Sage said the caucus are swallowing a dead rat.
"It's legislation that I don't like, but when you're in coalition there are some things you have to do as part of the coalition agreement," Ms Sage said.
"It wasn't part of our confidence and supply agreement, but it's important to another coalition party.
"It is a dead rat that we have to swallow."
The Greens are supporting the bill after failing to include waka jumping in a list of NZ First policies the Greens wouldn't support during coalition negotiations.
"Labour requested a list of NZF policies that we don't support, and while we went through, we didn't even think of the waka jumping bill," a Young Greens Facebook post relaying information from a phone call with James Shaw said in February this year.
National's Nick Smith says the bill is fundamentally undemocratic.
"They are openly saying they oppose it, so we are having changes made to our electoral law and our constitution without a majority of Members of Parliament supporting it."
"This goes to the heart of New Zealand's parliamentary democracy and is a permanent stain on the reputation of the Greens."