Yes, I was wrong
I spent the morning trying to chase down how the present regulations on passenger screening. You can find plenty of articles talking about screening of passengers on aircraft with 90 passengers or more, very little on how this screening actually came about. A bit about levying tickets to support the cost, lots of reports on the cost impact. There are some airports that do screening of turboprop passenger flights based on sharing of facilities, such as in Gisborne.
If I had to hazard a guess, the size threshold for screening is based on the anticipated value of the aircraft to terrorists. Small aircraft can't do as much damage as your 100 - 200 tonne passenger jets running into ground structures. One could also contemplate the calculus of loss of human life based on numbers, in terms of say investigative effort. Regulatory agencies tend to be measured on whether regulations were adhered to for small incidents, and there tends to be little public accountability on how regulations came about - before a large incident.
It's also hard to imagine prophylactic measures for in flight cockpit safety on small aircraft. There's been estimates as high as 600 million dollars for cockpit door reinforcement for passenger jets operating in the U.S. I'd imagine the cost for smaller aircraft would be disproportionately higher, weight and space problems might require an extremely large amount of re-engineering.
I'd expect an increase in passenger screening at small airports to eventually result from this incident. Air travel will be a bit safer in New Zealand, thanks to the great leavening of treating everyone as a potential risk.