The point about Ryanair in particular is they started with a very small aircraft in a very tight local market. They very nearly went to the wall, but a combination of animal cunning and creating a market for themselves saw them flourish.
The internal NZ freight market might be limited, but the international market is not and the demand for freight services is rising rapidly as internet shopping continues to flourish. I don't see anywhere in Kiwijet's announcement that they are limiting themselves to AKL-CHC or whatever. The 146 can reach across the Tasman, if it has the aux fuel tanks fitted. That certainly widens the possibilities somewhat.
If they manage to create their own market and revolutionise the industry a little, they could easily flourish. Airwork doesn't own the Air Post contract, a little bit of competition may well change the landscape.
Anyway - no doubt we shall see in due course!