Simpson, who has co-authored a book with Jennifer Cooke about the death of Tina Watson whilst scuba diving in Townsville in 2003, told Lawyers Weekly she is mystified as to the reasons why Gabe Watson's indictment for murder was downgraded to manslaughter.

"The closer you look at the evidence, the more convinced you become that this man is highly calculated, and very careful about covering his tracks," she said.

"That a jury has never been allowed to try the evidence ... is where I think the real travesty of justice lies."

Following an investigation into Tina Watson's death by asphyxiation, a coroner found there was sufficient evidence to charge Gabe Watson with her murder and try the evidence before a jury.

While he was charged and indicted for murder, a short time before he was due to appear in court to face the charges, he returned to Australia where he pleaded guilty to manslaughter, or causing death by omission.

But to Simpson, the fact that this occurred is astounding and does not fit with the evidence.

"The coroner said to me that he was not trying to find out whether a jury should convict, but whether the jury could convict. In his mind, conviction was certainly open to a jury. It is not up to him to worry about whether they would convict," she said.

But according to the DPP, the decision to downgrade the charge "was not taken lightly".

"The decision to accept Mr Watson's plea of guilty to manslaughter was made after a careful and thorough examination of the admissible evidence," said the Director of Public Prosecutions, Tony Moynihan, in a prepared statement.

"Given the complex circumstantial nature of the case, Mr Watson's admission that he breached his duty to render assistance to his wife ultimately meant there was no reasonable prospect of proving, beyond a reasonable doubt, that he was guilty of murder."

Gabe Watson is serving an 18-month sentence for manslaughter and is due for release in early November.

Claire Chaffey