There's certainly no major cultural figure that is the origin of the name. I've been vaguely aware of the name before, but if I thought anything of it I would have assumed that it was a creative attempt borrowed from the US. I had no idea it was specific to this part of the world!
The New South Wales Births, Deaths and Marriages search engine allows you to search recorded events from 1788 until 1907 (Births), 1977 (Deaths) and 1957 (Marriages). There are no Raewyns recorded in any of these. I'm stumped :-)