The 43-year-old woman extracted her own vigilante justice after discovering an older man had sex with her 13-year-old daughter.

The abuser has pleaded guilty to a sexual offence and is awaiting sentence in the ACT Supreme Court.

The mother can't be named, for fear of identifying a victim of a sexual offence.

Acting Chief Justice Richard Refshauge this afternoon sentenced her to six months suspended on entering a 12-month good behaviour order.

"As a human being, I can understand your reaction. But you, I think, understand that it was completely inappropriate," the judge said.

"At the end of the day, the criminal justice system will deal with the victim, and that's where it should be, in due course."

The woman pleaded guilty to assault occasioning actual bodily harm.

After her victim's crime came to light in December, 2010, she texted the man demanding they meet.

"Best you get your ass down now to talk about it, or else you're f**ked," she wrote.

The man came to meet her, and they and two unidentified people went for a drive.

At some stage the mother unclipped her seatbelt and punched him eight times in the head and back.

They pulled over and hauled him out of the car, abusing him, before driving away.

The court heard the victim suffered bruising and abrasions.

The next day the mother called police and reported the sexual offence against her daughter.

She told them she already "went and faced [the victim] and I flogged him".

The mother's lawyers noted their client suffered from daily seizures.

And they submitted the crime against her daughter had taken an emotional toll.

The court also heard the mother already had a criminal record, including a conviction for assault.

The judge said a sentence of imprisonment was required, but in the circumstances it could be served other than behind bars.