In the letter, Campion said she had been aware of Mander's book, but any issues concerning it were dealt with years ago as part of the process of reviewing and confirming copyright title before the film was financed.
She said the legal agent for the copyright owner of Mander's book had confirmed in writing it had no claims involving the book, screenplay or film.
A Queen's Counsel had also studied the book, the final screenplay, story outlines and other materials in 1991, two years before the film was released. It concluded there was no copyright infringement "and that any similarities were superficial."
"The only good that has come out of this is that Jane Mander and her book have become known to a greater New Zealand public who may be encouraged to read it," Campion said.
"I put a great deal of creative effort over many years into this work, as did many fine creative people. The awards that the film has received are a credit to us all, as well as to New Zealand.
"The continuing uninformed speculation undermines the efforts of everyone involved."