If the Mt Albert byelection was a practice run for Labour-Green co-operation in the election that matters most this year, it can probably be considered a success. Jacinda Ardern and Julie Anne Genter appeared to have nothing to argue about. Not a cross word was heard from either of them about the other. They looked and sounded so familiar voters might have been struggling to tell them apart.

If this is how they intend to campaign in the general election it is going to be good for Labour, possibly less so for the Greens. Labour will have no party chipping at it from the left and voters on the left will have no reason to support the Greens. They may conclude a Green presence would make very little difference to a Labour Government and therefore they should do what they can to maximise Labour's tally.

For it remains very important, even under MMP, for Labour or National to get more votes than the other. The winner of that contest has always been able to form a government. In the latest Colmar Brunton poll published by One News last Sunday, National had 46 per cent support and Labour 30 per cent. The Greens had 11 per cent so even if all of those voters went to Labour it would still leave Labour 5 per cent adrift of National.

It would need the support of NZ First (also 11 per cent), which it might get if the Greens were not in the picture. Winston Peters' aversion to the Greens saw them shut out of a previous Labour Government. But in that election, 2005, Labour narrowly beat National. Peters has always gone with the winning party when his preference has mattered, and probably would do so again.

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So Labour needs to do much more than cannibalise the votes of its natural partner.

It needs their partnership to attract more support than their combined poll numbers so far, or else Labour needs NZ First to attract enough votes from National to make Labour the election winner.

So far it is following the first strategy, using opportunities such as the Mt Albert byelection to show how amicable a Labour-Green coalition could be. But if Ardern has not got the result her party expected, that strategy could be reviewed.

There is, however, little Labour can do to advance NZ First's prospects of eating into National's lead. It has to stay with the Greens and hope their duet in Mt Albert will work on a national stage.