Forget any distinction between resigning and being sacked. Had Nick Smith not fallen on his sword yesterday, one would have fallen on him fairly promptly.

He had to go. He had to go because he has been silly or misguided or both in ignoring conflict of interest rules which apply to Cabinet ministers.

But he also had to go because John Key desperately needed to bring some closure to this untidy, unseemly episode and its cast of high-profile National Party figures and highly-explosive allegations.

The public may not know every last detail of the saga. But they will have seen enough for Winston Peters' categorisation of the goings-on as National Party "cronyism and sleaze" to start to find a ready audience.

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Such foot-in-mouth follies erode public confidence in a government. Key's and National's poll ratings are not so crash-hot that they give him and his colleagues unlimited latitude for mistakes.

What with opposition to partial state asset sales, the furore over sales of farmland to foreigners and other niggling issues, National has got more than enough of its plate.

Smith also had to go because the Prime Minister initially valued loyalty ahead of precaution.

Key mishandled things on Tuesday in letting Smith continue in his Cabinet roles after publication of the reference Smith wrote as ACC minister for his friend and National Party activist Bronwyn Pullar.

Key would have been better to have stood Smith down temporarily until he could be sure nothing else was going to emerge from the woodwork to embarrass him, Smith and the party.

So it was salutary and handy that a trawl of ACC files on Smith's instruction should yesterday uncover a further letter confirming his conflict of interest-loaded intervention in the case. The letter effectively crucified Smith because it reveals he asked ACC to provide him with information relating to Pullar's claim.

Such a request, in alerting ACC staff to the minister's interest in the case, might not have influenced how they handled it.

But the perception that it might had Smith in clear breach of conflict of interest guidelines in the Cabinet Manual.

Key simply declared that two letters was one letter too many. The day before he had taken a rather relaxed view as to whether Smith's reference breached conflict of interest guidelines. Yesterday they took on a sudden new importance.

Finally, Smith's departure will effectively allow Key to shut the door on any kind of inquiry into Smith's behaviour which might embarrass National. So Smith goes.

But his exit leaves Key far from unblemished.