...Wiki reports problems with weight and stability. 22 knots still seems a bit slow to me - and cruising's only 12. NZ sounds as if it's coping with a poor design, reading between the lines.
I agree. But the point remains that if all you want is a vessel suitable for law enforcement and patrol duties, anything much larger or more capable is a waste of time, manpower and money.
Something like what the US Coast Guard uses as a long-range vessel is probably a better design for the job, although it's verging on being a frigate itself given it's size and weaponry. It would of course be more expensive and seems excessively armed and equipped for the role we're considering. The CIWS alone is probably all the gun that's necessary for a ship conducting these jobs, if it ends up in a situation where more firepower than a 20mm rotary is needed things have probably gone beyond the point where a patrol vessel is much use anyway. You could also carry a few missiles or torpedoes for the helicopter, for those rare situations where you needed something with more range or punch.
Personally for a patrol vessel I'd prefer something smaller, although you do need a certain minimum size if you want to be able to operate a helicopter from it. As you point out, there's also stability and weight issues that smaller hulls have to face. Helideck at the stern, CIWS in the bow, crew of perhaps 50 all up seems adequate for the job we're talking about.