Doesn't holding that portfolio make it even more incumbent on her to follow the rules in the Cabinet Manual to the letter, and especially with regard to something as fundamental as avoiding any suggestion of conflict of interest, real or perceived?
Isn't it even more desirable that the Justice Minister set an example and be pure as the driven snow in ensuring there is no confusion of her official role with private business - rather than trying to argue that her dealings with the milk-exporting company Oravida did not create even the perception of a conflict of interest when her husband's directorship in the company leads inexorably and inevitably to such a conclusion?
Why did someone who is normally extremely savvy politically become so blinkered as not to see the distress she was causing to her caucus colleagues by refusing to admit she had made a mistake and apologise accordingly, thereby providing ample justification for Labour MP Grant Robertson to keep hounding her?
Above all, perhaps, can the National Party retain confidence in Ms Collins as a potential leader one day when she has shown such a flagrant and breathtaking disregard for the laws of politics?
The first law of politics is that often, the handling of a problem turns out to be as bad as or worse than the problem itself.
Whether Ms Collins now fully realises what an absolute hash she has made of things is still a very moot point.
As hard as it seemed for her to come to grips with what she had done, it is just as difficult to come up with a cogent explanation for her astonishing fall from grace.
The timing of the revelations of her indiscretions - and her refusal to acknowledge them until forced to do so by the prime minister - are said to have infuriated MPs.
Nothing else comes close to concentrating politicians' minds than their three-yearly appointment with the ballot box.
The onus is thus on Cabinet ministers and MPs to hold the line until polling day by projecting an image of competence and confidence while ensuring as much as possible that this picture is not clouded by foul-ups or embarrassments.
National MPs will be asking why Ms Collins failed to see how her actions were compromising this strategy - and whether, for all her undoubted strengths, there are some previously hidden flaws in her character which have now put her leadership credentials in serious question.
This is not an academic matter. Mr Key made it very clear this week that should he lose the September election, he will not be hanging around Parliament for very long afterwards.
Until this week, Ms Collins seemed to have all the attributes required to do the job - an abiding commitment to National Party principles such as individual responsibility, a diamond-hard resolve, a solid work ethic, an ability to capture the mood of the party, an understanding of what matters to the average punter, killer communication skills (especially when it comes to putdowns and razor-sharp one-liners) and, last but certainly not least, a capacity to deliver a steel toe-cap caress to the nether regions of National's enemies, always accompanied by that wicked smile that leaves no-one in any doubt as to who won.
The warning signs were there long before what might be dubbed the Oravida episode. Her projection of an image of being one tough cookie seems to have bred a feeling of invincibility - something which is fatal for a politician to contract.
It is certainly not a quality that sits comfortably with the compromises and flexibility demanded of politicians by the minority government model.
It is no accident that Mr Key has followed in Helen Clark's footsteps and been more of a managerial prime minister who basically tries to keep as many people as happy as he can for as long as he can.
Like Miss Clark, he has hugged the centre line, looking behind the enemy's trenches for areas to mine for votes, while pushing Labour ever leftwards.
Mr Key has proven that his strategy of territorial expansion of National's vote is best achieved by marching as much to the left as possible in a proportional electoral system.
Steven Joyce, the other main contender for the leadership, would happily follow that model. It is not so clear whether the more ideologically driven Ms Collins could or would do so, thus potentially making power more elusive for National.
This is important because the departure of an arch-pragmatist like Mr Key is bound to provoke some ideological soul-searching within National.
Ms Collins - who is unlikely to be in the dog-box for too long - could exploit her more clear-cut and less centrist beliefs to her distinct advantage if the party deems it needs ideological refreshment.
What is certain is that whoever succeeds Mr Key is more than likely going to have a hell of a time trying to stamp their mark on the job. Mr Key will leave super-size boots to fill.
You only have to look across to the Labour benches to see Miss Clark's legacy. She served as her party's No 1 for 15 years. Since her departure, Labour has had three different leaders in less than five years.
The longer Mr Key stays, the tougher it will be for his successor to come even close to matching his record. Ms Collins will recover her poise and panache soon enough. But National's next leadership ballot may well be a good one to lose.
- John Armstrong is The New Zealand Herald political correspondent.
