I would have thought Greylag geese would have been fairly easily obtainable in your neck of the woods because they certainly were when I lived in Taupo - although I didn't keep any at that time. They are abundant in the Far North.
However, before you get too involved in keeping geese, have your done your research on them? They are great birds, brilliant in orchards and gardens BUT breed on the smell of an oily rag, can decimate pasture and need severe culling when numbers get out of control - which can happen very quickly. Geese make very good eating so if you're wanting to get into them be prepared to cull heavily on an annual basis.
I keep a goose, a female without a gander and that's the way I want to keep it. She lives with the male Pekin duck (another very stupid breed), Paradise and Grey ducks. She's worse than the dogs at letting me know that somebody alien has arrived on the property.