By its proximity to us, I just cannot see the Yanks letting anyone attack NZ. (The question arises - like who would want to) but for argument's sake...
We might not agree with the US on many things (or even most things) but we do rely on them for support in time of need. It is an unfortunate fact of life. Our navy these days comprises one Manly ferry and a pilot launch (oh yes, I forgot, a fleet of submarines, very expensive submarines, that don't work under water), and we have no one else to depend on if it ever hit the fan. (The UK? hahahahahahahahaha!)
I'm waiting and hoping for some Kiwi input to this thread...!