Labour and the Greens want to present voters with a stable alternative government at the next election but they may have a problem - NZ First's Winston Peters.
The two parties on Tuesday announced they'd agreed to work together in the lead-up to next year's election, an unprecedented arrangement which could lead to a joint campaign.
But on current polling they're well short of winning enough seats to oust National and would need NZ First's support.
Labour leader Andrew Little said both parties had agreed "that this is not a monogamous relationship".
"This is not an exclusive arrangement, we will work together and others are welcome to join us," Mr Little said.
National's Minister Steven Joyce is playing down the agreement, saying there is "really nothing new here".
"Labour already can't say whether they prefer the Greens over Winston, or Winston Peters over the Greens, so that's where it started to sound like an episode of The Bachelor to me."
But Mr Peters says he has no intention of joining the arrangement.
"We don't like jack ups or rigged arrangements behind peoples' backs," he said.
"None of this is going to have any affect on me or my party whatsoever - we will campaign on our manifesto and on our policies."
At the joint press conference called to announce the agreement, Mr Little and Greens' co-leader Metiria Turei were asked what would happen post-election if Mr Peters demanded a deal with Labour that would keep the Greens out of government.
They didn't give straight answers to that, and wouldn't say whether the agreement would survive if Mr Peters wanted it torn up.
"We're not getting ahead of ourselves before the election," Mr Little said.
"Once the people have spoken, and the numbers are known, then we're in a position to talk about the formation of a Government."
The agreement doesn't commit the parties to joint policies or a joint campaign, but it's a strong indication that's what will happen.
And to demonstrate their togetherness, Mr Little is going to speak at the Greens' annual conference in Christchurch at the weekend.
"It's our intent to build on this agreement to offer New Zealanders the basis of a stable, credible and progressive alternative government at the 2017 general election," Mr Little said at the press conference.
Ms Turei said New Zealanders wanted a new government, and they wanted it to be stable.
"Under MMP voters want to know what they're getting," she said.
"This is crystal clear clarity."
The Greens sought a similar co-operative agreement with Labour before the last election, but Labour ruled it out because it didn't want to burn its bridges with NZ First.