It may be so that this current coalition disintegrates, that doesn't mean National would then be the next govt.
National had three coalition partners in the last Government.
Can you even name them now?
Two are dead and buried.The third is on serious life support.
There are a number of combinations of coalition that can maintain Labour as a govt, including as a minority govt. I think since the last election the political landscape is shifting. National have lost the 'shine' of being in power (the authority, or the appearance of power). Depending on your point of view, most of their cabinet look like they are in the twilight of their political careers and have that smell of desperation for profile / publicity.
Labour are riding the whole fab around 'non-white, non-male' etc etc, and whether or not you agree with it, it is a 'thing'.
There is a generational shift going on in politics, stale pale and male is so last year now...
In short, if the coalition does collapse, I wouldn't count on National being able to form a government. They may, but it definitely isn't a surity solely on the coalition collapsing.