Polynesian religious knowledge was transmitted orally, with precision, by people trained to hold it, and the schools that trained them were among the first institutions to be disrupted by conversion and colonisation.
The nineteenth century produced the bulk of the written record, and it was produced under pressure. Missionaries collected in order to replace. Colonial officials collected in order to administer. And in several islands, indigenous scholars wrote down what they held, sometimes because they judged it was about to be lost. The Hawaiian and Maori manuscript traditions of that period are the single most valuable body of material, and they are also products of a moment in which the tradition was being fought over.
That produces a specific distortion. Some accounts were shaped by their collectors toward a tidy single mythology, occasionally with a supreme creator inserted to make the tradition look more respectable to Christian readers, and arguments about how far that happened, particularly with the Maori figure of Io, are long-running.
The tradition's own carriers were also selective, and legitimately so. A recitation belongs to a line, and some of it was not given to collectors at all.
Archaeology, linguistics and the study of navigation have since supplied an independent frame, tracing settlement across the Pacific and confirming relationships between traditions that comparison of names alone could only suggest.