I purchased this book in the 80’s from a major Australian book chain. I don’t remember the author or title of the book.
In the book, the protagonist is a young girl whose mother is a witch. She rules some sort of kingdom under the sea.
The novel ends in a battle between the protagonist and her mother, which the girl wins. I think they battled using thunderbolts. I remember a large thunderbolt striking the mother which ends the battle. The mother becomes good again and I think she reunites with the girl’s father.
The protagonist also has a brother in the novel but I can’t quite remember his role.
I can also recall a description of sand dunes.