News last night was that New Zealand prime minister Helen Clark has moved to expel one of her MPs, Taito Phillip Field, who is under investigation for bribery and corruption. Apparently Field’s threat to stand as an independent was the last straw:

[Labour] has cut a lot of slack and you claim you’ve been neglected by the party and you felt so badly about it that you might stand for somebody else at the next election. Well, at that point, we’re not fools.

Most governments wouldn’t be too troubled by the loss of just one MP, but New Zealand’s democratic voting system makes things rather more interesting.

At the last election, in 2005, Clark’s Labour party won 50 seats, two ahead of the opposition National Party, (Adam Carr has all the figures). The remaining 23 seats were shared among six parties: New Zealand First (7), the Greens (6), the Maori Party (4), United Future (3), ACT (2) and the Progressives (1).

Sign up for a FREE 21-day trial and get Crikey straight to your inbox

By submitting this form you are agreeing to Crikey's Terms and Conditions.

This posed something of a challenge. Imagine Kevin Rudd having to negotiate a coalition simultaneously with the Greens, Family First and One Nation and you get something of the idea.

What emerged was a formal coalition with the Progressives, and an agreement with New Zealand First and United Future under which their leaders got seats in cabinet but weren’t bound by collective responsibility, and in return they promised to support the government on issues of confidence.

A quick calculation shows that gave Labour a bare majority, 61 seats out of 121. The Greens, who had marketed themselves during the campaign as the most reliable partner for Labour, weren’t needed, so they got left out.

They weren’t happy about it, either, co-leader Ron Donald saying at the time that Labour’s deals were “socially, economically or environmentally destructive”.

The Greens did, however, agree that they would abstain rather than oppose the government on votes of confidence. Now, with the loss of Field’s vote, that commitment suddenly becomes important.

Clark said “she had double-checked last night the solidity of that arrangement” — presumably before burning her bridges with Field.

This is a key advantage of proportional representation: majorities are usually so small that even if one side has the numbers, it can’t take the other groups for granted, since things might change.

John Howard has already learned that with his narrow Senate majority; now Helen Clark is getting the same lesson.