There are a few key points which IMO are inarguable. The first is that the RNZN has not been properly resourced in a generation. One of the unfortunate follow-on effects of that, is that the NZDF has been forced to rob Peter to pay Paul, in order to acquire, or in some cases, maintain a capability. It becomes a question of which capabilities have a greater impact, a higher priority, or a lower cost.
One of the other points which I consider inarguable, is that the assigned mission for the IPV's has been changed from when the design was commissioned as part of Project Protector. A follow-on question for that, is whether the IPV design is really appropriate for extended operations in the seas around NZ while far from shore. I suspect that while the vessel might be fine, the crew would not be, due to sea state and other conditions.
Another related follow-on question is why the change in vessel role? I certainly do not know, but I suspect it was because Gov't deemed patrols/intercepts conducted further out of greater impact or higher priority than those done inshore. If the goal is to stop illegal foreign fishing vessels, then I would suspect those would not be operating all that close to shore. Also for local fishing vessels, those can be handling by land-based agents, since the vessel would need to land their catch at some point.
Two final thoughts: As I understand it, NZ has gotten away from measuring patrolling in sea-days, since patrols are supposed to be more queued or vectored by other assets, (a la the P-3K2's). Which I suspect have been observing more suspicious vessels away from the NZ coast, as opposed to either just over the horizon or within sight of land. Side note; the sea-day expectation for the IPV seems to have been unrealistic IMO. 290 days out of the year, means nearly 80% at sea. Quite apart from being potentially hard on a small crew, that would only leave about 20% of the time for training, maintenance, etc. I cannot imagine that there are sufficient Customs and Fisheries officers that one would be aboard each IPV 290 days out of the year.
My second thought here is that the specifications for the IPV's at least, seems to have been largely influenced by Customs and/or Fisheries. While I do think a whole of gov't approach can be good, in that non-Defence agencies can gain benefits from access to Defence assets, it can also be take too far IMO. When the primary owner/user of an asset is selecting something which is suitable for another department, but not particularly suited to their primary role/mission, then the asset selection becomes questionable. A vessel design with the expectation that 1 in 6 crew aboard are not Navy sounds more to me like a vessel paid for, owned by, and crewed/operated by Navy for the benefit of other departments. If those other departments have a legitimate need for an asset like an IPV, why do those departments not seek to get the funding to acquire and sustain/operate one?