I don't know where the NEAQ and PIPA are getting that little bit of information, or how Tukabu "knew there were cats on this island", but we've spent a whole lot more time on the island than either of them and as far as I know, TIGHAR has never in run across a live cat on Niku in 9 expeditions out there. I'll put the question to EPAC and find out for sure.
There are little polynesian rats on all parts of the island. They are more like big brown mice. They prey on the crabs, and the crabs (and probably some birds) prey on the rats, so I don't think Niku needs cats to keep the rat population in check. They do, apparently, get quite aggressive during the dry spells.
The rats were there before the British brought the colonists. Trying to eradicate them now would be quite some project. Might have to import some cats, then owls to get rid of the cats, and then colonists to get rid of the owls, then......
Andrew McKenna