Now only the key word ngoio remains to consider:

Presumably it qualifies hatu in a way similar to how ika qualifies taotao. If so, then ngoio should refer to some quality of the newborn Moon. I have found only one reference in Churchill regarding ngoio:

Certainly the new moon is comparatively dark, and its crescent is small.

Birds should characterize the 'path of growth' mapped by the steep northwestern corner of the island up to the bay of Anakena. The image of a small black bird is fitting for a station when Moon is yet only a small baby.

Moreover, we should remember there is a black fish on the other side of the island, viz. Te Pe'i. It is located to the west of Vaihu, in a position which could be said to correspond to that of the black bird Ngoio: