Kiwi readers will already know that former Immigration Minister and Labour Party MP Philip Field was yesterday convicted of corruption. Long story short, he took bribes from poor Thai workers, in the form of unpaid labour on his houses, in exchange for his facilitating their visas. I have little to add beyond what Stephen Franks and David Farrar have already said on the issue.
The bigger scandal for me isn't the corruption; it's that the previous government did everything it could to prevent any kind of serious investigation of charges against Field. Do check the above Farrar post.
Helen Clark couldn't easily fire Field for the same reason that she couldn't fire Winston Peters, whose false declarations of political donations earned him Parliament's censure and should have cost him his position as Foreign Minister - Peters was "stood down" but never fired, letting him keep the Ministerial Baubles of Office.
Labour's coalition governed by a rather narrow margin. In a 121 seat Parliament, Labour (50 seats) plus NZ First (7 seats) plus United Future (3 seats) plus Jim Anderton (1) adds up to 61 seats. Clark kicked Field out not for corruption, but because he signaled running for a new party in the next election. Kicking him out meant that the supply and confidence agreement with the Greens became rather important; it's not implausible that Labour was forced into supporting Sue Bradford's anti-smacking bill because of this. Had Clark done the right thing and kicked Peters out, she would have lost all of NZ First, requiring her to get both the Greens AND Maori on side.
And so we have another pernicious effect of MMP: encouraging corruption, or at least massively discouraging its punishment. Governments in First Past the Post systems tend to be strong enough to withstand losing an MP or two if need be. Not so under MMP; we expect minimal winning coalitions. And so another chalk-mark in favour of John Key's building a broader coalition: he can credibly kick anybody out, and even lose either all of ACT or all of the Maori Party's support, and still govern. While this means that ACT cannot really constrain National against silly things like forcing the superannuation fund to invest ridiculous amounts domestically, it also has upsides.
Kiwis like to go on about their clean image. Transparency International ranks us well, at least for now. I don't expect nonsense of this sort under the current administration, mostly because Key has arranged things such that he cannot be in the pinch Clark was in. We'll see what happens after the next election.
Update: Farrar notes that Labour was winning confidence votes by a margin of about a dozen at the time (ie, Greens and Maori Parties tended to vote with the Government). It does make a difference, though, whether the party's block vote is inframarginal or marginal. In the latter case, the negotiating position changes; wishing to avoid such a change may well have contributed to Clark's not sacking Field.