She had to make a judgments based on policy positions as to what would be problematic. That one slipped through because it had been so long since similar legislation was before the House.
But the bill is causing a headache. It has made the Government look arrogant as its ministers swat away the views of academics and lawyers up and down the country.
It is causing public divisions within the Green Party as well it should, being such a dog of a bill.
The allows caucuses to fire troublesome MPs from Parliament, undermines the basic freedoms of MPs to speak freely, and increases the reach of the courts into Parliament by making caucus decisions reviewable.
It also has the potential to create tensions between New Zealand First and the Greens.
But that has been limited to some extent because the Greens have been rewarded for swallowing the dead rat of the party-hopping bill.
In February, Ardern surprised media by suddenly announcing a policy change on the Bill of Rights Act.
It seemed a really odd move in the early days of a Government to elevate a policy that was not part of any coalition or confidence and supply agreement. It was the policy of only the Green Party but even then, not a priority.
The change means that if courts declare a law to be inconsistent with the Bill of Rights Act (BORA) 1990, Parliament will be required to review that particular law.
In the perpetual contest of power between Parliament and the courts, it will also give the courts more influence over Parliament.
It was unexpected manna for the Bill of Rights Act warriors in legal and academic circles.
For New Zealand First leader Winston Peters, who has been a black letter lawyer all his political career, it was the sort of move he would normally oppose, if only to prevent a new generation of "grievance" lawyers emerging on the back of it.
But Peters swallowed that constitutional dead rat, the Bill of Rights Act move, and the Greens swallowed theirs on the Electoral (Integrity) Amendment - with a BORA dessert to follow.
The parties will be reluctant to openly admit they have been horse-trading over lofty constitutional matters – but sources have confirmed they are linked.
It was not so much as a deal as a way of saying thank-you to the Greens.
The Bill of Rights move was a way of Labour and New Zealand First recognising the pickle the Greens unwittingly found themselves as a result of an oversight by Labour.
So why can't the Greens simply oppose the bill?
There are two reasons: first because the final clause of both agreements with Labour says that the Greens and New Zealand First will act in good faith to allow Labour to comply with the other party's agreement.
For the Greens to oppose the bill would be a technical breach of the agreement.
But the more important reason and one that the wider Green membership may not appreciate is the consideration of relationships within the Government.
For as long as the bill remains crucial to Peters, the Greens blocking it would corrode goodwill in that relationship. Goodwill within the three-way Government is crucial.
If Peters were a less obdurate politician, he would recognise that the problem was created unintentionally by Ardern - a good faith error.
He would see that persisting with a bill to address a non-existent problem in Parliament is creating unnecessary grief for the Greens and ask Labour to withdraw it.
But that is unlikely. Instead, the parties are persisting with the notion that proportionality on election day is sacred and must be preserved except, of course, when Winston Peters wins a byelection, as he did in 2015, and gives his party an extra list MP.
Peters had trouble 20 years ago with mass defections of MPs from his party. He has not had a defection since then.
His caucus expelled Brendan Horan from the caucus but that was for matters related to his personal life, and it reflected on New Zealand First's candidate vetting. Peters has fallen out with other MPs, but they have been demoted down the party list.
The best thing the Greens can do, if they won't oppose the MP-firing bill, is to give it an expiry date.
One of the more interesting factors in the debate is why the former Green MPs are so passionately opposed to the bill compared to the current crop who genuinely don't seem to mind it.
New MP Golriz Ghahraman in the first reading was bragging about having "improved" it by making it judicially reviewable - seemingly oblivious to the significance of inviting the courts into caucus rooms.
Dissent is not as important to the current crop of Green MPs. Their party is in Government, their issues have become mainstream. Dissenters are the two MPs they wanted to get rid of last election or are conservative rabble on the other side of the House.
The generational difference of view within the Greens may be because the likes of Jeanette Fitzsimons, Keith Locke and Sue Bradford have exercised dissent all their political lives and in more hostile environments than today's.
An unfettered right to dissent is a fundamental principle worth fighting for, more than a concept of proportionality, which has never been pure anyway under MMP.
On this occasion, the pragmatism of Greens inside Parliament may well beat the principled stand of those outside.
And passing a bill to fire MPs is never going to lose anyone votes.
