Also in NZ, according to the official dictionary I checked.
In fact all sources I've looked at agree that "boo-ee" is strictly US.
I don't know how present Canadian usage goes; "boy" is my native
pronunciation, but I certainly heard "boo-ee" in my youth -- perhaps
only from Americans. None of the dictionaries give any information about
the distribution of the two pronunciations within the USA.
And where did the Americans get the "boo-ee"? Pace J.J., it's a fairly
good approximation to the Dutch -- perhaps an independent borrowing via
NY Dutchmen? Or maybe just spelling pronunciation?
Which brings us to the origin of the peculiar spelling, on which we all
agree. Whence? It's not based on any of the languages from which the
word might have been borrowed.
I append some vaguely suggestive notes from OED's etymology:
"It is not clear whether the English was originally from Old French, or
Middle Dutch. The pronunciation /bwɔɪ/, indicated already in Hakluyt, is
recognized by all orthoepists British and American; but /bɔɪ/ is
universal among sailors, and now prevalent in England: Annandale's
Imperial Dictionary, 1885, has /bɔɪ/ or /bwɔɪ/, Cassell's Encyclopædic
Dict., 1879, says ‘u silent’. Some orthoepists give /buɪ/."
The "bwoy" pronunciation may be a clue; OED gives it as (still) an
alternative for BrEng.