Her 26-year-old abuser, Nathan Roberts, will spend up to five-and-a-half years behind bars for his offending, after admitting four charges of sexual penetration of a child.

Roberts wasn't violent to the girl, but consent laws and legislation protecting children under 16 exist for good reason, Judge Michael McInerney said on Tuesday.

There's a need to protect young women from men who have sex with them when they're underage and particularly when they do so without using protection.

"It's hard to imagine how stupid one can be, but here we are," he said of Roberts.

"As a result of such stupidity and criminality we're in this position where Mr Roberts is before the court (and) the victim has a child to look after."

The sexual relationship between the pair was short-lived and it's believed the girl became pregnant after their second time together.

It began in August 2020 when the girl told Roberts she was only 15.

They had sex on four occasions, including once when she was heavily intoxicated. Roberts was also drug and alcohol affected at the time.

In late-August the pair again discussed her age and the risks of him having sex with a child of her age.

"Apparently a form of agreement (was made) where she said she wouldn't tell anyone of the circumstances of their sexual liaisons," Judge McInerney said in the County Court.

The girl later told her mother she believed she was pregnant. A doctor who confirmed the pregnancy reported Roberts' offending to police in line with requirements.

Roberts admitted his actions to police. He was sentenced as a serious sexual offender because of the ongoing nature of their relations and will be placed on the sex offender register.

The girl told the court that she did not realise what was wrong with the relationship at the time because of her young age.

"She clearly loves her child very much, but particularly she recounts the restrictions and difficulties on ... a young girl having the obligations to raise a child," Judge McInerney said.

"The restrictions she talks about are exactly the restrictions which form the basis for parliament passing this legislation."

Roberts must serve at least three years and nine months before becoming eligible for parole.

Judge McInerney wished Roberts luck for the future and said he had good prospects for rehabilitation.

He was supported in court by his mother and partner, who broke down after the sentence was handed down.

Australian Associated Press