Collective defence especially in the maritime domain is more than direct territorial threats to sovereignty. SLOC, presence, intel ....I don't believe that NZ would have to face threats to its sovereignty on its own.
Anyone posing a major threat that country will also be posing a threat to Australia in my opinion.
NZ won't be on its own
MB
With a direct threat to NZ or Australia neither Navy could cope that is why we have had the BFG out there with us since the 1940's. For us of course some days have been better than others in the past.