Sure, the convenience of attending larps in New Zealand is limited to a smaller set of people than in Europe. My calculations are very loose.
Europe is complex. I know that The Gathering in the UK attracts people from other parts of Europe. I imagine the two massive German yearly fests with 5,000 players each probably attract some people from places like Switzerland, Austria, etc. I have the impression the vast majority of people attending these events are nationals (because of the convenience of location and common language), but no proof of that.
An easier, and very encouraging, example is Bicolline in Quebec. I understand that it is largely attended by French-speaking people from Quebec (i.e. it is linguistically isolated as all materials are in French and the surrounding states are English-speaking), which has a population of around 8 million people. Bicolline has a lot of events through they year, the largest of which attracts 2000 people. This is the basis on which I calculated a possible maximum of 1000 people in New Zealand, which has half the population of Quebec.
Of course, some of my assumptions on Bicolline may be out. Bicolline has a massive medieval venue which must attract a lot of people who otherwise might not attend. And I imagine Quebec would have easier travel than NZ, because it’s blob-shaped and we’re long and narrow. On the other hand, nearly a third of NZ’s population is located in one city and NZ’s fourth largest city is a short trip away. Still, even if 1000 people is twice as optimistic as it should be that still leaves an estimated feasible event size of 500 people.
Consider Mordavia. We had little difficulty attracting 60-70 or so people twice a year. When we attracted interest from an enthusiastic high school player, we would suddenly be indundanted with their mates as well. Consider all the people who never even heard of Mordavia. Given the pull that we got from the small amount of marketing we did, I think 300 people would be an easy number to achieve if approached artfully and persistently.
NZ’s largest living history event NAAMA attracts at least a couple of hundred people. In Europe at least, I think numbers at large larps are comparable to living history numbers or outstrip them. In the US the opposite seems true, there is at least one massive living history event but no massive larp. I think it’s reasonable to guess we could equal or exceed the size of NAAMA with an event.