Sacking a senior Cabinet minister just three weeks out from election day is never a good look - especially when you are accusing your opponents of being the ones in disarray and inherently unstable.

John Key's worry must be that the whole dirty politics saga steadfastly refuses to die. But then there is not much the Prime Minister can do about that.

What Key has done is swiftly exercise prime ministerial fiat and removed what had become National's weeping sore in this election. And that is absolutely crucial. The public wanted Collins gone. And finally she has.

- LIVE UPDATES: Judith Collins' resignation
- JOHN KEY: It was the honourable thing to do
- EXPOSED: The email that brought her down
- PHOTOS: Judith Collins - her career in pictures

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If the election campaign was showing signs of turning against National, Collins forced resignation should help shift it back in the governing party's favour.

Not initially, though. There will be an Opposition-instigated firestorm for the next day or so designed to paint Key as weak. But as almost always is the case, sending a minister to the backbenches sends the problem with him or her.

Watch: John Key announces Judith Collins' resignation

The Opposition's biggest target has been removed. There will be argument as to whether Key has seized on an email which intimates she was undermining the chief executive of the Serious Fraud Office as an excuse to get rid of her. Or whether the contents of the email are so disturbing in terms of the minister-chief executive relationship that Collins had to be stood down while the matter is investigated.

What is not in question that cumulative impact of the Oravida scandal, revelations in Nicky Hager's book Dirty Politics followed by the emergence of the email left Key with no choice but to demand Collins' resignation.

Whoever sent a copy of the email to the Beehive would have been conscious of that. Christmas has come early for the prime minster.

What the Opposition must now do is to convince voters that the unsavoury political culture detailed in Hager's book is endemic in the Beehive and not restricted to just one or two offices.

As far as Key concerned, he will be asking one question: is there more yet to come?

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