There was land/islands inbetwixt Australia and South America that sunk by earthquake and tsunami.
Uh, no. I've read only one tale (Vincent Gaddis) of a lost island in the Pacific, it supposedly sank (earthquake, or tsunami, or eruption), and used to lie due west from northern Ecuador, or maybe it was the Isthmus, and that one (assuming it's a true tale) went under in the 18th c. There are good solid reasons to regard the tale as a really compelling work of short fiction though.
A quick look at the map of the subsurface Pacific shows a number of seamounts, none of which are close to the the surface today, but at one time all of them were above the former sealevel of the Pacific. They are also not close enough for "coast-hugging", a practice that is largely a modern invention of landlubbers, the same origin IOW as the foolish claim that until Columbus, everyone thought the world was flat. Those seamounts sometimes occur in a bunch, but even when the oceans were hundreds of feet lower (and that was long before the Polynesians arrived) the Pacific was mostly a blank, featureless expanse.
The navigation abilities and traditions of the Polynesians survived into the 20th century sufficiently long enough to be documented and studied, and probably linger here and there to this day, but for the most part died out a generation or so ago. Maritime peoples throughout history and geography relied on fishing, and navigated by sun and stars in order to seek their prey.
The kon-tiki raft built by Heyerdahl mimicked some South American traditions. The Easter Islanders themselves had (maybe still have) origin stories involving non-human ancestors such as whales, but I very much doubt that anyone seriously believes those. Their ancestors had a period of internecine warfare (mostly nocturnal) after the collapse of the statue-builders, and hid in caves, during which time they lost much of their former culture.
Native Balsa Raft
Sketch by F.E. Paris (1841) showing construction of a native balsa raft from the north-west coast of South America. The maximum length of raft is 80-90 feet, maximum width of a raft is 25-30 feet with a freight capacity of 20-25 tons.
TOR HEYERDAHL BALSA WOOD RAFT - BRADSHAW FOUNDATION link