It may indeed be historic but the agreement between the Green Party and Labour may also effectively have handed the 2017 election to National on a platter.

Labour's leader Andrew Little was at pains to emphasise the new memorandum of understanding between the two parties was "not monogamous".

The trouble is that the one man needed to form Little's Big Love government is NZ First leader Winston Peters and Peters prefers monogamy.

The agreement itself is pragmatic. It provides room for the parties to 'agree to disagree' in a respectful fashion and to work together where they can.

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From the Green Party's point of view it makes sense. It shows they can work constructively and provides some insurance they will not be tossed aside as cavalierly as 2005, when NZ First demanded it.

The agreement does make it harder for Labour to ditch the Greens but it also effectively ends on election day. Little was very careful not to give any guarantee if Labour needed Peters and that was the price of getting him.

Green co-leader Metiria Turei was at pains to insist things had changed in the 10 years since Labour last jettisoned the Green Party for Peters, saying political leadership and the landscape had changed since then. That is true, but the one leader who has not changed in all that time is Winston Peters. Peters takes great pride in not changing and he is not about to start now.

Whether the agreement makes sense for the Labour Party is another question. It did not need the agreement - it was already a given that the two parties were natural coalition partners.

It is certain there was some resistance in Labour's own crew to the development. Little has made claims of supporting 'middle New Zealand' in recent times and the perception Labour is aligning too closely to the Greens risks undermining that.

There was a grin on Peters' face after the announcement for a reason. Labour's support base includes a significant chunk of working class voters who identify more with Peters than the Green Party. Peters will be betting he can scoop up some of that support from Labour. He has already begun, accusing both Labour and the Greens of selling out their supporters.

It is no secret Peters - and some Labour MPs for that matter - think the Green Party is toxic for Labour's chances of Government. Nor is it any secret that if a Government can be formed with him alone, that is exactly how he likes it.

The only person who will be most delighted by today's turn of events is one John Key, Prime Minister, whose chances of retaining that title just increased without him having to lift a finger.