[comment caption=Do you think there is a gap between what Labour says and what it does regarding sustainability issues?]Rarely has the Labour Party been given a dressing down from a supposed ally like the one Jeanette Fitzsimons' dished out in Parliament on Thursday afternoon.
Her scathing denunciation of Labour's record on climate change and other environmental matters would take some beating in terms of its savagery and sarcasm.
The Greens' co-leader hurled question after question at the Prime Minister to clinically, but brutally, expose the gulf between Labour's "sustainability" rhetoric and the more mundane reality.
Ms Fitzsimons asked how credible Helen Clark's facade of carbon neutrality and claim of "world leadership on sustainability" was when greenhouse gas emissions had jumped by 14% since Labour came to power.
She asked how credible that facade was when 75% of new electricity generation came from non-renewable thermal fuels, the Department of Conservation has just sacked 60 staff and the Government took pride in having the largest road-building programme the country had ever seen.
Labour is used to periodic eruptions from Parliament's cross-benches. But this one had more than the usual bite.
It reflected annoyance that Labour has captured the language of climate change politics but has been getting away with failing to deliver the goods.
Coming six months out from the election and on the eve of their annual conference, however, this was also a case of the Greens getting bolshie ahead of a possible power shift on the centre-left.
The Greens face the best of worlds and the worst of worlds.
There has never been a time when their issues have been so relevant, and minor parties tend to do well when the fortunes of the major party on their side of the political spectrum are on the wane.
However, though the Greens may well return to Parliament in bigger numbers, the aftermath of this year's election must look distinctly unattractive from their perspective.
National will probably dominate the next Parliament in numbers not far short of a majority.
To govern, National could conceivably need the Greens to abstain on confidence motions, but propping up a National-led Government is still outside most Greens' comfort zones.
If the election result is closer the Greens, with the help of other minor players, may be able to prop up a fourth-term Labour-led Government.
But that administration would be hugely unpopular if it was seen as being cobbled together simply to thwart National. It would probably be too unstable to last.
The Greens will campaign as a totally independent party.
The decision on which of the two major parties they might seek to have some relationship with after the election will be based on an assessment of those parties' respective policies.
This is markedly different from the situation three years ago, when the Greens declared they wanted to be part of a Labour-led Government and would not be talking to National.
However, Labour's post-election spurning of the Greens caused deep hurt.
That has persisted, despite the relationship being patched up by a co-operation agreement which saw Labour agree to implement a long list of policy concessions in return for the Greens abstaining on confidence motions.
That anti-Labour undercurrent surfaced during the repositioning exercise with unanimity across the party that it adopt a more "independent" stance - as in "independent from Labour".
Any independence is illusory, however.
The Greens are hamstrung by being to Labour's left. Simple ideology means they have to work with Labour.
Even if the Greens dropped their drive for "social justice" and shifted to the centre they would still lean far more heavily Labour's way. Labour realises this and behaves accordingly.
While National and the Greens maintain channels of communication, the likelihood of a post-election deal remains extremely slim.
Not only are many Greens loath to deal with National on principle, they are petrified that even talk of co-operation will see the party's left-leaning supporters take flight.
National's likely first preference will be a deal with the Maori Party.
It is conceivable that National and the Greens could agree to co-operate on policies where they share the same goals as a tentative first step to something bigger.
But electoral mathematics suggest that if National needs the Greens, then Labour will also be a player in post-election talks.
Even if National wins more seats than Labour, the odds are heavily stacked on the Greens favouring the latter. But at a price.
Labour has effectively shut the Greens out of Government three elections in a row.
The warning underlying Fitzsimons' attack on Thursday is that the Greens will this time be asking a very high price for their co-operation.
With NZ First and United facing electoral slaughter, they could be in a position to do so.
When it comes to price, Ms Fitzsimons is not referring to the baubles of office.
The Greens may judge it wiser to stay outside the Government, while demanding policy undertakings and possibly holding ministerial portfolios under similar arrangements to those that Winston Peters and Peter Dunne have with Labour.
What Ms Fitzsimons was really flagging is the Greens' intention to hammer out a deal under which Labour will be held to account so the reality of climate change policy starts to match Labour's boasting.
John Armstrong is the New Zealand Herald's political correspondent.
