Realistically, there are only a handful of countries now that could seize NZ and end sovereignty. NZ is just too remote for almost every country in the world to be able to project sufficient power to invade and then hold ground, never mind subjugate the populace.
Of those nations, NZ is either friendly with or allies of most of them. For the nations which is neither friendly or allied with, the world community generally reacts with hostility to one country conquering or even attempting to conquer another. Also, those same friendly or allied nations which could project the requisite power would IMO react particularly badly, and given that three of them are permanent members of the UN Security Council with veto power and nuclear weapons...
IMO what is are more feasible and likely threats to NZ security and interests. These would be man-made events which harm or exploit NZ resources, circumstances which adversely impact NZ access to trading partners, events within the borders of trading partners which adversely impact either the function of the market or need/relevance of Kiwi goods within that market.
Among the least likely of events would be a direct strike on or against NZ. Part of my thinking this is because of the remoteness and difficulty which makes projecting power so difficult, also makes carrying out a strike at such a distance very difficult. More countries could accomplish a strike than an invasion, but it still is not a long list and in most scenarios there is not a nation with sufficient animosity and/or potential gain to launch such a strike. That and the world community generally and NZ's friends and allies would again not take kindly to such an attack.
To provide some additional context regarding the difficulty in accomplishing an invasion and subjugating the populace consider the following. Given the effectiveness of past insurgencies, it can take an occupation force on average 7-10 troops per insurgent to effective suppress them. NZ per the 2013 census has a population of 4,242,048. That means if just 1% of the populace were to actively resist an occupation, that would be a force of ~42,000 resistance or guerrilla fighters. That would also mean the aggressor nation would need to be able to deploy an occupying force of 296k - 420k troops and then sustain that force over the very long SLOC to NZ.
IMO there is really only a single nation which could accomplish that at present, and unless the leader of that nation suddenly decides they want to own a golf course, resort, or some other such real estate, there is little danger of invasion from them...