A retired Catholic priest has avoided being sent to jail for sexually abusing a teenage girl more than 50 years ago when he was in charge of a WA home for Aboriginal children taken from their families.

Key points: Mithen sexually assaulted the teen in 1965 while in charge of the mission

Mithen sexually assaulted the teen in 1965 while in charge of the mission After the assault another priest told the girl she had sinned and would "burn in hell"

After the assault another priest told the girl she had sinned and would "burn in hell" The judge accepted Mithen's remorse and subsequent community contribution

Allan John Mithen, 80, pleaded guilty to two charges of indecently assaulting the girl when she 15 and 16 in 1965 at the Wandering mission, about 120 kilometres south of Perth.

She had been removed from her family when she was four years old and taken to the mission, where the District Court was told she was sexually, physically, emotionally and psychologically abused.

Mithen's crimes happened just after he was appointed as the superintendent of the home when he was aged about 26.

Teen told she would 'burn in hell'

The court heard that after the abuse, Mithen told the teenager she was bad and had go to confession, which she did, and was told by another priest she had committed "a mortal sin" and would "burn in hell."

Mithen's lawyer, Ian Hill QC, submitted his client was remorseful, ashamed and embarrassed about what he had done, saying he had otherwise lived "an exemplary and useful life" and devoted himself to helping those less fortunate than himself.

He said Mithen had joined the priesthood as a young man, almost as a way of retreating from the wider community, because he was shy, withdrawn and unable to deal with relationships.

The offences took place at Wandering Mission south of Perth. ( ABC News )

Mr Hill said Mithen was emotionally immature and ill equipped to run the Wandering mission, although he maintained he did make some positive changes including the introduction of payments for children at the facility.

He also apologised to the woman in the 1980s and gave her about $4,000 dollars to help her when one of her children was suffering medical problems.

Mr Hill said Mithen — as well as being elderly — was in very poor health and suffered from a range of illnesses including one that required him to have blood transfusions every three weeks.

Teen's 'innocence stolen'

Judge Laurie Levy described the offences as a significant breach of trust, saying Mithen had manipulated circumstances so he could be alone with the girl and abuse her.

He said the victim was young, vulnerable, innocent and naive, and Mithen had stolen her innocence.

"One can only imagine the intense turmoil she went through ...," Judge Levy said.

But he accepted the 80-year-old was remorseful, and that since the offences he had made a significant contribution to the Catholic community and the lives of many indigenous people, particularly in Sydney where he had worked with the community in Redfern.

The court was told Allan Mithen was suffering from a range of illnesses. ( ABC News: Joanna Menagh )

Judge Levy sentenced him to a jail term of 13 months, but said the combination of mitigating factors meant the term should be suspended for 15 months.

The decision means Mithen, who returned to the eastern states after his time at the Wandering Mission, will be able to return to his home in Victoria.

Supporters of abuse victims angered by sentence

Before the sentencing, supporters of Aboriginal child sexual abuse victims conducted a silent vigil outside the court, which included a smoking ceremony.

After the sentencing they expressed their anger that the jail term was suspended.

Curtin University senior Indigenous research fellow Hannah McGlade said the victim was a vulnerable Aboriginal girl and Mithen should have been sent to prison.