Scenario 1: China decides it's not enough to antagonize Japan, Taiwan, Vietnam, Malaysia, Singapore, Indonesia, Philippines, Brunei, and by proxy, the US in its neighbouring waters (plus the usual beef with India), and wants to start building artificial reefs in NZ territorial waters and/or inserting "polite people" who then unanimously vote to join North Island into China.
Scenario 2: Putin decides Russia needs a proper tropical colony and decides to do the same, once the Pacific Fleet is made great seaworthy again.
Scenario 3: Hardline islamists seize power in Indonesia and they decide to spread the jihad to NZ because it's easier to conquer than the infidel lands in the immediate neighbourhood.
Scenario 4: The cows came home along with the flying pigs and the ever-menacing cargoship full of Argentinian revanchists accidentally lands in Auckland instead of Port Stanley. Hilarity ensues.
The thing most working for NZ is the sheer distance from anything resembling a threat, and as they still have a functional Army merely reaching the islands is not enough, one would have to be able to maintain a supply chain over most of the Pacific. Of course, outlying islands (Tokelau and Cook Islands) are still at risk, but would not really be defensible even with combat aircraft unless there would be a permanent force stationed there. (Also, to reach those islands the potential attacker would have to pass several US-held islands, which might attract undue attention to the invasion fleet.)
However, if China starts to become a proper blue-water navy, Russia gets its act together again, or Indonesia indeed starts flexing its muscles, then NZ should of course evaluate their options. Gripen E/F would probably be a good choice, though its range tends to be on the short side. (Or indeed Gripen M, if RAN decides to become a carrier force again the NZ Gripens could then cross-train on Aussie flattops...) Eurofighter is then the next logical step, but the cost may be prohibitive.