He used their bedtime stories as a chance to groom his 9-year-old daughter, reading to her before climbing into her bed and raping her.
But without the seven months of offending coming to light, he stopped knowing it was wrong.
At the man's sentencing at the North Shore District Court today for charges of sexual violation by rape and unwanted sexual connection, the judge and an experienced barrister called his ability to end it "highly unusual".
They both said normally sex offenders were discovered and forced to stop instead of ending it by themselves unprompted.
However, Judge Lawrence Hinton said the 45-year-old's offending was "horrendous and utterly unacceptable" given his victim's extreme vulnerability as a child who he and his wife took into their care from Child Youth and Family when she was 18 months old.
The court heard how in September last year the man, who has permanent suppression on all identifying features to protect his victim, started asking his daughter for a cuddle after reading her a bedtime story.
"Then the cuddling progressed where you would rub the girl's legs and buttock area with skin on skin contact."
This happened about twice a week.
By December the offending had worsened to a point where he would take off her pants and rape her.
He told her to keep it a secret.
This continued through until May - a total of nine or 10 times - when the man realised what he was doing was wrong, told her this and stopped the offending.
However, it came to light when the girl told her mother who confronted her husband.
The court heard how he went straight to the police, confessed and detailed the offending so they didn't need to launch an investigation.
The man pleaded guilty at the earliest opportunity and accepted responsibility for what he'd done and was very remorseful, his lawyer Guyon Foley told the court.
Foley said in all his years as a barrister he'd never come across a sex offender who'd stopped undiscovered and said it may even have been "unique".
"He didn't try and roll the dice and wonder, 'Can I get away with this?'."
But Crown prosecutor Taylor Bellingham said the "enormous breach of trust" by the man, the age difference of 36 years between him and the girl, the prolonged period of offending and that their bedtime stories were an opportunity for grooming were all aggravating factors.
Judge Hinton agreed that it was "highly unusual" for a sex offender to stop their behaviour before it was uncovered and credited him with pleading guilty as early as he could.
However, he said the man needed to be held accountable and his offending had to be denounced.
"There was clearly premeditation. You abused access to your victim's bedroom nightly for bedtime reading. Your victim was vulnerable. She could not be, I suggest, more vulnerable," Judge Hinton said.
"You were her father by adoption by whom she is entitled to expect safety, care and love and not abuse. The abuse she received from you was the utter antithesis of her expectations of you."
He sentenced the man to eight years in prison for both charges to be served concurrently.