What is this book called?

For context I am Australian and would have read this childrens book when I was 7 in 2004. I was an advanced reader so this could be a book for older kids too. I have never remembered what it was called and it haunts me to this day.

What I remember of the story was that there were kids and an island of ghosts and an old man (who I think was alive and not a ghost but I'm not certain). A plot point I remember was that many years ago the old man had a young woman called Amelia come to live with him to be his wife and then later there was an earthquake or something similar and he thought she ran away with all of his treasure which is why he was so mad. However, the kids later find her body in a cave so it is revealed that she didn't run away, she was trying to protect his treasure but got trapped/crushed by rocks and died instead.

What is the name of this book?