It has to be said that if losing one minister is a misfortune, losing another looks like carelessness.

Yesterday John Key fronted yet again on the resignation of a support party minister - this time Act leader John Banks.

Key had little choice but to accept Banks' resignation, just as Banks really had little choice but to offer it.

Key has made much of holding his ministers to high standards and a failure to do that for Banks would have reeked of political convenience.

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Key's primary aim yesterday was to make reassuring noises about the stability of his Government. Those assurances may be true, at least for now. Banks has pledged to support National on confidence and supply. United Future leader Peter Dunne did the same when he resigned as a minister after refusing to hand over emails to the Prime Minister's inquiry into the leak of a GCSB report to Fairfax reporter Andrea Vance.

Nonetheless, the ministerial resignations of both Banks and Dunne does reinforce the perception that the stability is as wobbly as blancmange.

Key will be relying on those assurances more than he is letting on. He has pointed out he can still muster a majority with the Maori Party. But the Maori Party votes against far more government bills than it supports, and that leaves vast tracts of National's policy platform at risk of being put on ice. It would still be able to govern, but only in first gear.

Politically, it is atrocious timing for Key, who had hoped to spend this time trying to be a pernicious gooseberry on new Labour leader David Cunliffe's honeymoon.

Instead, this imbroglio will help the budding momentum Labour has built up. It won't take much for Cunliffe to play on that perception of instability, and suggest National is now relying on a ragtag bunch of minor players with question marks over their honesty.

The timing also adds to National's longer-term headache - future coalition mates. If Banks does appeal against the pre-trial decision and that appeal fails, it will simply have served to drag proceedings out and could mean he still has a trial hanging over his head heading into next year's election. It would be untenable for Act to stand him as a candidate. Even if Banks is cleared, it will be hard for Key to justify asking Epsom to continue to vote for him in 2014.

As for Key, he will be hoping another adage doesn't come true - that bad news comes in threes.