The mechanism is called "donations protected from disclosure", and it has seen nearly half a million dollars funnelled to right-wing parties this election cycle (weirdly, no left-wing party has ever received a cent). It was initially introduced as an ugly compromise in Labour's Electoral Finance Act, to cover their failure to fully eliminate secret donations. And while National repealed the EFA, it kept that part of the law because it was useful to them. The theory is that the donations are made in total secrecy, so there can be no influence - and if you believe that, I have a bridge over Cook Strait to sell you. What it is in practice is the government laundering money for large donors who wish to exercise their influence in secret, or do not wish to suffer the reputational damage of their political views. It is, in effect, a state money laundering scheme to hide corruption.
There are two fundamental principles underlying election finance law: the rich must be prevented from buying government, and any funding that is permitted must be totally transparent. This government money laundry violates both. It allows secret influence, and worse, makes it a crime to expose it. That should not be permitted. This corrupt law must be repealed.