Q:

What is the name of the island used in Ocean Girl?

What is the name of the island used in Ocean Girl? Is it a real island in Australia?

A:

As far as I know, Neri's island was never named in the series, so I doubt the in-universe location can be identified with a real location.
Out of universe, scenes set there were filmed on High Island, part of the Frankland Islands in North Queensland. From OceanGirl.org, which is an excellent resource for all information about the show:

Most of the outside/island shots were taken on locations ’round Port Douglas and as “Neri’s Island”, High Island, which is part of the Frankland national park Islands of Cairns, Queensland, Australia.

