Well, we've got Emmarentia! Which is presumably named after some long-ago
Emma Rens or
Van Rensburg -- it's an old suburb of Johannesburg, just down the road from me, and it's caught on and become a standard name! I was at school with a girl named Rentia (her full name, and a putative short form), and there was an opera singer a few decades back who made it "Italian" and called herself
Emma Renzi.
But if there isn't a link between Narellan and
Narelle, then surely they must at least have, or have had, a common ancestor? Perhaps from some now extinct local language?