The Greens are determined to be part of the next government and Labour probably won't have a choice about that, but it won't be a marriage made in heaven.

Helen Clark was a clever politician and during her nine years as prime minister she spurned the Greens, choosing NZ First and United Future as her partners after the coalition with the Alliance collapsed.

There were good reasons for that and they still exist.

The Greens are a left-wing party and Clark knew she had to move Labour towards the centre.

David Shearer is doing that now, seeking the middle New Zealand vote that put National into power in 2008 and returned it last year.

Clark didn't want the tensions a coalition with the Greens would bring, and she didn't want to have to accommodate extreme policy demands she was sure would come from them.

She had choices, Shearer probably won't.

The Greens are much stronger now, with 14 MPs against the five or six they had during Clark's reign.

It's inconceivable Labour could put a majority together without them in 2014, assuming National doesn't cling on for a third term.

The Greens came out of their annual conference last weekend reaching for the sky.

They think they can get more than 14 MPs next time and they'll hold Labour to ransom if they have to.

The Greens will want cabinet positions, they will want to control environment policy, they will want a big say in economic policy and they will want to promote social policies which are a long way left of Labour.

And they will want their brand to be up there in lights, because they've been around long enough to know what happens to parties that go into coalition with Labour or National.

They are swallowed up, they don't get the credit for what little they're allowed to achieve, and voters forget they exist.

The Greens don't consider themselves to be a minor party and they're not going to let that happen to them.

There could be significant potential for disagreements and instability, which would damage Labour's longer term ability to stay in office.

The way Labour and Green MPs are talking, the next election is theirs for the taking.

That's partly because the government has had a horror start to its second term, and the opposition parties know that winning a third term is very difficult under the best of circumstances.

But they still have a lot of work to do.

The Labour/Green vote in last November's election was 35.54 per cent. National won 47.3 per cent.

Opinion polls haven't changed much since then and there still has to be a big swing to the centre-left before a Labour-led government ousts National.

When the numbers are crunched, NZ First is the devil in the detail.

The party came back with 6.6 per cent of the vote last time, if it holds on in 2014 Winston Peters could be calling the shots.

A three-way Labour/Greens/NZ First coalition is nightmare territory and it probably wouldn't run a full term without collapsing.

Whichever way you look at it, Labour is going to have a tiger by the tail if it manages to stop National winning a third term.