A man who waited until the third day of a trial for raping his de facto stepdaughter to plead guilty has been resentenced after a group of judges determined his jail time was "manifestly inadequate".

The initial 10-year sentence he received in August 2022 has been set aside by the NSW Court of Criminal Appeal, which has imposed a 12-year sentence with a non-parole period of eight years.

The man cannot be named to avoid identifying the girl he abused, who has since died.

The girl was 11 when she and her mother moved in with the man, who was unemployed and living on an inheritance from his parents.

He soon began engaging in "disinhibited and sexualised conduct" towards her, which continued for a number of years, as he dismissed concerns raised by other family members.

"Unsurprisingly, the young girl was scared of (him)," Justice Fabian Gleeson wrote in the court's decision published on Tuesday.

She was "half-asleep" in July 2020 when the man came into her bedroom and raped her.

After he unlocked the door and left, she cried herself to sleep.

The next day he acted like nothing had happened. She began locking her bedroom door.

Less than two weeks later, she went into her bedroom to find the man already there.

He aggressively raped her again, mumbling something like: "If you tell anyone I'm going to ..." on his way out the door.

The girl did tell someone in September 2020, and the man was arrested that November.

He waited more than a year, until the third day of his trial in February 2022, to plead guilty, after a jury had begun hearing the case, including evidence suggesting he had a tendency to act in the way he was accused.

The offender had tried to blame drugs for his "heinous conduct", and any expressions of remorse he had made were "more about the impact upon himself", Justice Gleeson wrote, summarising the sentencing judge's remarks.

Justice Gleeson said the sentence imposed by Judge Andrew Coleman, SC, "did not adequately meet the strong need for general deterrence in relation to sexual offences committed against young teenaged girls by persons in a position of trust or authority".

The court has previously noted "the prevalence of aggravated sexual intercourse without consent by stepfathers against girls in their early teens", making such deterrence necessary, Justice Gleeson wrote.

The offending also resulted in significant harm to the girl, which was not accurately reflected in the sentence.

Justice Desmond Fagan, in agreeing with Justice Gleeson, noted there was no specific error in the calculation of the man's sentence

"However, each of the indicative sentences was in my view excessively lenient, by a significant margin, and this has led to the aggregate sentence being manifestly inadequate," he said.

The man will be eligible for parole in November 2028.

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