Australian Bull Ray

Actually, the Australian bull ray is found around NZ! I realise now that the page I linked you to shows the distribution as only around Australia, but that's not correct. I linked the page because it had lots of info on the species, didn't see that the range was not quite right!

A little difficult to make out from this picture, but from what I can see it has that "blotchy" pattern typical of the southern bat ray (also called Australian bull ray). Given your location, I'd say that's the only likely candidate! Very neat that you observed orcas eating them! The link I'm providing in the ID only has a few pictures, I'd encourage you to do a Google Image search as these rays are very varied and you'll find some individuals that match this pattern more! The page I'm linking to has tons of info though.