It was when a teenage boy entered a confessional booth to reveal the Catholic sin of masturbation that Melbourne priest Peter Waters started "grooming" him.

Father Waters, in his 30s at the time, was a talented piano player and writer, gregarious and charismatic, teaching one of his young charges how to drive and taking him on an overseas holiday.

As their "spiritual mentor", he befriended underage boys at the parishes he worked at - and their families too - including the child in the confessional.

Through the partition, Waters recognised the boy, addressed him by name, and asked about his sexual practices. The boy felt like he was encouraged to continue masturbating.

Prosecutor Robyn Harper said it was the 1980s when the boy stayed overnight with the priest.