The Greens have a stiffly worded message for Labour. If the latter thinks its post-2011 election relationship with the Greens will revert to the "big brother, little brother" status of the last decade or so then Labour needs to think again.

In a provocative, yet important, scene-setting speech on Sunday, Greens co-leader Metiria Turei stressed her party's success in last year's election meant its relationship with Labour had to be one of "equals" from now on.

In gaining 11 per cent-plus of the official party vote - up from just under 7 per cent in 2008 - the Greens had their best result yet. In contrast, Labour's share of the vote fell from about 34 per cent to below 28 per cent.

There is still a vast gulf between the two parties in terms of support. Turei's demand for equal treatment will consequently be viewed by Labour as little more than posturing.

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What she was really saying was that Labour could no longer afford to adopt a "take it or leave it" attitude towards the Greens - a stance which saw Labour deliberately shut the Greens out of power-sharing arrangements through the 1990s.

Labour needed to grasp that the political landscape had changed. That those voters backing the Greens last year would not automatically return to Labour in 2014. That the Greens were now a political force generating support in their own right from quarters that Labour's policy prescription couldn't reach.

Turei's speech reflects another reality. The party is once again more likely to end up exercising power with Labour rather than National.

First, propping up an unpopular third-term National Government post-2014 is not an enticing prospect.

Second, National's second term has already been marked by a noticeable drift to the right which will make it even harder for the Greens to strike a deal with the governing party.

The Greens will continue to have a far more limited memorandum of understanding with National to allow the two parties to work together on areas of common ground - as they did in National's first term.

But Turei flagged the Greens were likely to be far more vehement in their criticism of National on matters where the two parties disagreed.

Until now, the Greens have been more comfortable with half of the traditional Opposition role of "oppose and propose", preferring the latter in order to appear constructive.

The Greens are conscious that on some subjects - foreign investment in New Zealand and state asset sales - Labour is compromised by its record in Government. So the Greens feel an obligation to criticise the Government more vigorously on such matters.

Far more significant, however, is another reality of current Opposition politics. The return of Winston Peters to Parliament plus the continued presence of Hone Harawira make the Opposition benches crowded territory.

To hold on to their 11 per cent of the vote - let alone boost it - the Greens know they must be much better at basic Opposition politics. That means being more hard-edged, taking advantage of new Labour leader David Shearer's relative inexperience, and keeping a high media profile. It's a simple choice - swim with other Opposition sharks or sink from view.