Arguably there is the matter of oceanic separation, but this is also true of Tasmania, an island the size of Ireland. Why did New Zealand and Fiji decline, whereas Tasmania opted to join?
There is much, much more to say, but a useful starting point may be my answer in this thread.
An important bit of context is that when I wrote that answer, the thread was full of people helpfully explaining that the reason New Zealand did not join was because of the Treaty of Waitangi, a traditional explanation that I find wanting. Hence my focus on New Zealand's own particular racial anxieties.
A fuller answer would focus more on New Zealand's gradual disinterest in the Federation Conferences of the 1880s and 1890s, with a focus on Premiers Ballance and Seddon. However, that thread should give you something to go on in the meantime.
I will endeavor to remember to provide a fuller explanation of New Zealand and Australian plans for the annexation of Fiji tomorrow.
Did Papua New Guinea fall into the same group?