Another worrying element is that Mr Banks might have given us a glimpse of a practice that is common in political party fund-raising.
He had been a National MP for many years before he retired and won the Auckland mayoralty. He may have been fund-raising for his mayoral campaigns in the same way that he had seen parties raise money for parliamentary elections.
Anonymity of big donors is probably more important to the parties than the donors. The parties stand to suffer more than their benefactors if the relationship becomes known.
This case warrants a reconsideration of campaign finance law to require the naming of all contributors of more than $1000 to a candidate or party. We no longer need listen to claims that private donations would dwindle and that much more public funding would be required.
But in the meantime, what should happen to Mr Banks? The best the Prime Minister can say for him is that Mr Banks believed he was acting within the law.
The law, it bears repeating, states an anonymous donation is one "that is made in such a way that the candidate concerned does not know who made the donation". If Mr Banks did know he was getting a donation from Dotcom, as Dotcom says he did, his position is untenable.
The incident arose during his last mayoral campaign, not last year's general election at which he won the Epsom seat in Parliament for Act. But he would not be the first MP to have to resign for an indiscretion before he stood for Parliament.
His departure would create a byelection in Epsom that National would need to win if the Government was to retain its majority.
Labour has called for his suspension pending a police investigation of a complaint Labour has filed, but the police should not be put in this position. Electoral law is a matter between those elected and the public they have asked to trust them.
Mr Banks must have known the identity of his big donor and did not disclose it. He has lost the public trust and should go.