The man, who cannot be named for legal reasons, was sentenced to two-and-a-half years jail in Bega Local Court on Tuesday after pleading guilty to four counts of indecently assaulting the teen who was in his care at the time.

Dressed in a black suit, the man wept as details of his offending were read to the court.

Magistrate Doug Dick said the man's offending, which included touching, kissing and biting the girl's breasts and "grinding" his genitals on her through clothing, could have a lifelong affect on his victim.

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"Sadly and even tragically she'd been your stepdaughter for most of her life. Those who take part in this conduct must expect to go to jail," he said.

Magistrate Dick said the man was drinking heavily in the period leading up to the offences in March and April last year, continues to "struggle with guilt", and is of a below average risk of reoffending.

"Sexual abuse inevitably leads to psychological issues," he said.

He said the man should continue to receive Christian counselling through his church, as well as through Alcoholics Anonymous.

Director of Public Prosecutions lawyer Alice Pinkerton described the man's offending as "predatory", "opportunistic", "not isolated" and a "breach of trust".

She said the man had said he was "flattered by the victim's attention", and pushed for a sentence which would "adequately denounce this kind of offending".

The man's lawyer unsuccessfully requested a short delay in his imprisonment to allow time to complete programs, which he said would lower his risk of reoffending, and hinted his client may appeal his sentence.

Magistrate Dick said the man was unable to show his detention was not justified, and agreed with the prosecution's submission he may be a "significant flight risk" due to the sentence imposed. He said programs were available in prison, and full-time custody was the only option.

The man's lawyer also raised the issue of a text message sent by the victim to her boyfriend saying she was "good at lying and being persuasive", however also stated the comment could not be linked to the accuracy of her victim impact statement.

The man will be eligible for parole in July 2020.