Christopher Saunders hosted "bunga bunga" parties at church properties and spent thousands of dollars of church money on cash payments, mobile phones, alcohol and cigarettes, Seven Network and The Australian newspaper have reported.
The bishop has denied the allegations and has not been charged by Western Australia Police but he resigned in 2020 after sexual misconduct and bullying claims emerged.
The 200-page Vos Estis Lux Mundi investigation, the first in Australia, identified 67 Aboriginal boys and young men who may have been subjected to sexual assault or grooming by the bishop.
It's understood that some or all of the alleged offences happened in other states or territories.
"It has been established through the interview of witnesses and examination of documentary and other evidence that Bishop Christopher Saunders has developed a modus operandi of grooming young Aboriginal men for sex during his time within the Kimberley region, both as a Priest and as Bishop," the report said.
It said interviewed witnesses described the bishop as being "a wonderful orator and strong leader" but also a sexual predator preying on vulnerable Aboriginal boys and young men.
The report was also critical of the WA police investigation into the bishop's alleged offending and the decision not to press criminal charges due to difficulties in establishing the ages of victims at the time of the alleged offending.
WA Premier Roger Cook said it was a "blight" on the Catholic Church, that the incidents had not been reported earlier.
"I'm pleased the Vatican has undertaken this inquiry and investigation," he told reporters on Tuesday.
"We want these situations, these investigations to get to the bottom of the matter so we can find some redress and some comfort for the victims of this sort of sexual abuse."
He called on the report's authors to provide any new evidence to police for potential use in a prosecution case
WA Police said it had requested a copy of the Vatican's report and if further information comes to light it will investigate.
The force said it had carried out two investigations into a number of complaints regarding a member of the Catholic Church in the Kimberley region between 2018 and 2020.
There was insufficient evidence to lay criminal charges.
"Offences of a sexual nature are complex and challenging to prosecute," a spokeswoman said.
"In all instances, the welfare of complainants is of the utmost importance."
In a statement Perth Archbishop Timothy Costelloe, who is president of the Australian Catholic Bishops Conference, said the allegations against Bishop Saunders were "very serious and deeply distressing" and it was proper they were thoroughly investigated.
He said the church's investigation was overseen by Brisbane Archbishop Mark Coleridge and entrusted to an experienced and independent specialist investigations organisation.
The report was provided to the Holy See, with the Dicastery for the Doctrine of the Faith continuing the investigation, Archbishop Costelloe said.
"We will respect the enduring confidential nature of this process by not commenting on specific allegations that have been raised," he said.
"In due time, the Holy See will make its determinations. It is hoped that this will not be unduly delayed."
Archbishop Costelloe said that only after "a just and authoritative finding" had been made could the process of rebuilding the church community in Broome under the leadership of Bishop Michael Morrissey continue to make progress and bring healing.
No new complaints have been made to WA Police since its second investigation was completed.
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National Sexual Abuse and Redress Support Service 1800 211 028
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