The problem that the Greens present for Labour is that they are more red than green. They have chewed off a great chunk of votes from Labour's left flank while scaring centre voters away from Labour to National. The Greens squeeze both ends of what was Labour's voter base.
National's problem is that none of its sure partners is successful enough. Labour's problem is that its sure support partner is too successful.
The Greens' very success runs the risk of denying them the possibility of government. A vote for Labour is a vote to put the Greens in power.
The Greens provide National with an easy strategy this election. National can wheel out the mad, bad Green Party policy and declare it the would-be Labour-led government's.
That serves to scare even more voters National's way and put Labour on the back foot.
Labour can't ignore the claim. The Greens are polling too high for that. Accepting the policy makes Labour look weak which, in turn, drives more votes National's way. Rejecting the policy upsets Labour's left-wing membership and activist base, drives more left votes to the Greens and risks a bun fight with the Greens.
It would seem logical for the Greens and Labour to sit down ahead of the election to establish policy agreements and disagreements and a broad-brush approach to ministerial positions. But politics isn't logical.
The Green membership would conclude their MPs were sellouts, giving away the bulk of their policy before the election and Labour would risk losing more centre voters.
Centre-right voters are smart. They know to vote tactically to ensure National has the numbers.