I don't know if any of William's, or any of the suspects', families or acquaintances are linked to NZ in any way ... I certainly haven't heard or read anything to that effect previously.
Travel between New Zealand and Australia for citizens of both countries is easy. Australian citizens do not need a Visa to visit. The number of Australians moving to New Zealand has increased in recent times (
http://m.nzherald.co.nz/business/news/article.cfm?c_id=3&objectid=11583862) - highly unusual as it's previously been the other way (New Zealanders moving to Australia to chase the big bucks and good life). This is relevant to this thread, as it shows that nobody would think twice of a little Australian kid accompanying an adult (or adults). And yes, traveling the seas between New Zealand and Australia is commonplace, as is traveling to any of the Pacific Islands from either country. New Zealand and Australia are both effectively islands, with an incredible amount of coastlines that a boat could sail into (although, Harbour Masters in both countries would no doubt be on the lookout for any arrivals).
William would now be five years of age - so if he was living in a built-up area (where other people lived and could see and hear him), he would be expected to be going to school, otherwise he would be reported for truancy (that is, if he is in New Zealand).
This is interesting ... I just found a NZ Crimestoppers page for William - I'm not sure how long this has been up:
https://www.crimestoppers-nz.org/media-release/missing-child-william-tyrell
and - this is more interesting - note this paragraph from his Wiki page:
"In early 2015, two passengers and a member of the New Zealand-bound flight crew thought they saw William on their airplane. The police met the aircraft at the airport and soon discovered it was not Tyrrell."
https://en.wikipedia.org/wiki/Disappearance_of_William_Tyrrell
I'd never heard of this sighting before. I wonder what it was that led to whomever thinking it was William, and how it was definitely ruled out that it was him? We must bear in mind that William might be using another name now.