The victim's family were in court for the sentencing.
In submissions, defence counsel Bill Dawkins said Watson had admitted his wrongdoing and had sought help from a psychologist, not to assist with his sentencing, but to gain some insight into why he had committed these acts.
Until the point of offending, Watson had lived a laudable lifestyle following Christian principles, Dawkins said.
"Why does someone who's ostensibly, barely in his 60s, lived this good life ... behave in this abhorrent way?
"In [the year the offending started] ... his good conduct ceased.
"His reputation, everything he built up, stopped."
Judge Walker said the summary of facts was nine pages long.
"At 67, you have not previously appeared before the court. Although it's very clear you have been leading a double life for several years now."
While many people in his life had abandoned the defendant, he was still getting support through church associates, Judge Walker said.
A victim impact statement revealed how Watson's offending had affected her life.
Judge Walker said the victim felt isolated and now had a warped sense of what a loving and trusting relationship should look like.
Due to her age when the offending began, she did not realise what he was doing to her was wrong.
"It took a huge amount of courage for her to come forward to say what had happened to her," Judge Walker said.
A Department of Corrections assessment tool and a psychologist's report both stated Watson posed a low risk of reoffending.
Watson was also in ill health and had recently had significant surgery. As a result, his jail term would have a disproportionate impact, and as such Judge Walker gave some discount to Watson's final sentence.
Judge Walker sentenced Watson to five years and one month's jail and ordered him to pay $15,000 for emotional harm reparation.
He did not give Watson a non-parole period, leaving it to the Parole Board to decide when he should be released.
Where to get help:
• If it's an emergency and you feel that you or someone else is at risk, call 111.
• If you've ever experienced sexual assault or abuse and need to talk to someone call the confidential crisis helpline Safe to Talk on: 0800 044 334 or text 4334.
• Alternatively contact your local police station
• If you have been abused, remember it's not your fault.