A WA author says she has come across an "unbelievable error" in the 1975 murder mystery of Shirley Finn.

The brothel madam was shot four times in the head as she sat in her Dodge at the Royal Perth Golf Club in June 1975.

Shirley Finn Credit:Facebook

Author Juliet Wills wrote a book about the case and is close friends with Ms Finn's daughter Bridget Shewring.

The case is currently before the WA Coroner.