The sacred ibis, a close relative of the white ibis that was formerly treated as being the same species (if I remember correctly), has gone feral in parts of Europe, most notably in France, and unlike most introduced bird species, it has certainly earned its title of being a major ecological pest.
This is because it predates on tern eggs and chicks, and terns being much smaller than ibises, they cannot effectively defend their nesting sites. A document that I no longer have access to described ibises being observed in the process of destroying every egg and chick at a single colony; if I remember correctly, the number of ibises involved was very small, and they finished off the eggs and chicks of the colony in a matter of hours.
That's just as bad as the damage dealt by mustelids and cats to tern colonies here, and similarly, the ibises are being culled there (in fact, I haven't found anything regarding whether or not any currently remain there).
New Zealand has two highly-threatened endemic terns; the black-fronted tern, and the NZ subspecies of the fairy tern.
I don't know whether or not the Australian white ibis predates on tern eggs and chicks, but the fact that it's very closely related to the sacred ibis alone is enough for me to assume that it does.
Obviously, what I'm saying is that Australian white ibises colonising New Zealand may be exactly what NZ's endemic terns don't need, especially when the threat presented by cats, mustelids, rats, etc. is still far from gone.
No, I am not saying that any Australian white ibises that reach NZ should be shot on sight, as in what happens to Canada geese and house crows in Australia.
What I am saying, though, is that this is a species that has the potential to have a major negative impact on at least two of NZ's endemic birds, and that this should be kept in mind.
It's a perfect example of how just because a new coloniser in New Zealand is good news for birders, it isn't necessarily good news for other birds.
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