By bathing in a river on theisland of the great Havai'i, young Terehe has defied the law. The angry gods brought up an eel from the depths. This agitated eel devoured a piece of land, causing the separation of the great Havai'i and the birth of Raiatea and Taha'a. The eel, now the Tahiti Nui fish, was then taken to another destination and became an island.

The great Havai'i
In ancient times, Raiatea and Taha'a formed a single large island called Grand Havai'i. One day on this island, the priests set about building a new marae and proclaimed a tapu (a ban). No noise is to be made so as not to disturb the sacred atmosphere. No roosters may crow. No dogs may bark. No one may move about.
But a young girl from the royal family of Raiatea, Terehe, a rare beauty, breaks the sacred law and bathes in the river. The irritated gods send a tunapu, an eel from the depths, out of a hole and swallow Terehe whole.
But Terehe's soul disturbs the eel's spirit and it descends into the depths. Losing its balance, the eel swims in all directions, leaping about tearing up trees and rocks. It devours the middle of the island. The result is a strait that separates the great Havai'i into two distinct islands: Raiatea and Taha'a. The eel grows larger and larger, transforming itself into an enormous fish. The god Taaroa sends the high priest Turahunui to guide the fish to its destination.

The fish Tahiti Nui becomes an island
The fish takes the name Tahiti Nui (the great Tahiti). When the fish arrives at its destination and is stabilized, no warrior is able to free it with his axe, despite their courage and valour. Tafa'i-upo'o-tu, the hero with a dignified heart, is called in and offers to go to Tubuai to fetch the sacred axe from the great chief Marere Nui. Tafa'i returns with the supernaturally powerful axe, but it's so heavy he can't lift it. Tafa'i invokes Tinorua, the master of the ocean. The axe becomes light and he can easily break the bonds of the gigantic fish, now an island.
This large, beautiful fish became the beautiful island of Tahiti. Orohena, the highest mountain, is, as its name suggests, the first dorsal fin. Tahiti iti (little Tahiti, the peninsula) and Moorea were the second dorsal fin. The islands of Me'etia (Mehetia) and Teti'aroa are the fish's droppings when it stopped at Hitia'a.

SOURCE: TAHITI HERITAGE