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''Untruthful'': Detective Chief Inspector Peter Fox whose claims instigated the inquiry. Photo: Darren Pateman

The ''hero cop'' whose explosive claims about NSW Police handling of Catholic Church child abuse helped spark a royal commission, has been branded a liar in a report handed to the state government on Friday.

Detective Chief Inspector Peter Fox claimed that a police investigation into child sexual abuse allegations in the Catholic diocese of Maitland-Newcastle was a ''sham'' and asserted there was a ''Catholic mafia'' in the NSW Police that covered up crimes.

He also claimed he was forced to ''stand down'' from the investigation into allegations surrounding two paedophile priests who had abused children in the Hunter area for decades.

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Late Bishop Leo Morris. Photo: Supplied

The Special Commission of Inquiry into whether the Catholic Church covered up or NSW Police failed to properly investigate paedophile priest activity found there was no evidence to support Chief Inspector Fox's claims.

It also found the response by senior church officials into abuse claims made against Father Denis McAlinden and Father James Fletcher, both now dead, was ''inexcusable''.

The 750-page report by Commissioner Margaret Cuneen found that Chief Inspector Fox had ''developed what amounted to an obsession about both the Catholic Church and alleged conspiracies involving senior police''.

Monsignor Patrick Cotter

Monsignor Patrick Cotter. Photo: Supplied

It found that evidence he provided to the commision was ''implausible'' and ''deliberately untruthful''. It also found that he was prone to ''exaggerate aspects of his evidence''. ''The commission formed the view that Fox had engaged in conduct that was inconsistent with the integrity required of a police officer,'' the report noted.

Chief Inspector Fox, who is overseas, has vigorously defended his actions, saying he would ''walk the same path'' if such matters came before him again.

In an exclusive interview with Fairfax Media, Chief Inspector Fox took a swipe at the commission, suggesting much of his evidence had been distorted and that the commission served more as a witch-hunt against him than an investigation into the cover-up of child sexual abuse.

''I am saddened by the process and findings, but do not shy away from my comments of 2012,'' he said. ''Throughout the special commission I felt more like a criminal on trial than a witness.''

Central to the inquiry was Chief Inspector Fox's claim that Strike Force Lantle, established in 2010 to investigate alleged concealment by church officials of child sexual assault by clergy, was a ''sham'' that had been ''set up to fail''.

The commission found the investigation was of a ''high standard'' and the officers assigned to the task were competent in their duties. It also found no evidence of a ''Catholic mafia'' within NSW Police that was obstructing the investigation of claims against clergy.

''The commission finds no credible evidence to support the notion that there are senior police … who were prepared to take steps to try to ensure that alleged child abuse offences involving Catholic Church officials were not investigated or not properly investigated,'' it noted.

However, the commission described the conduct of senior clergy, including the late Bishop Leo Clarke, former head of the Maitland-Newcastle Diocese, and the late Monsignor Patrick Cotter, as ''inexcusable''.

It found that information supplied to the police by Bishop Michael Malone, head of the diocese from 1995 to 2011, was both ''late and inaccurate''.

General secretary of the Australian Catholic Bishops' Conference, Brian Lucas, was found to have failed to protect children in attempting to have McAlinden resign from the ministry rather than reporting him to police.

The commission also found there is sufficient evidence to warrant the prosecution of a senior church official.