The boy pleaded guilty to one charge of sexual intercourse with a child under 10 years in Queanbeyan Local Court.

Defence lawyer Michael Bartlett said the boy was "prepared to plead guilty to one count of the offence on the basis the full facts were presented".

According to police documents tendered to the court, the offence carries a significant penalty which could include life imprisonment.

The boy, who was 14 at the time of the alleged offence, has been removed from the home he shared with other foster siblings in the Queanbeyan region.

It is alleged the incident occurred last year when the boy offered to read his foster sister a book in his bed.

It is alleged in court documents the boy asked if the victim wanted to "do the secret thing" and "play with each other's privates".

The boy had sex with the victim before a foster parent found the children hiding beneath the bed covers without their pyjamas on.

The foster parent took the victim from the room. It is alleged she said, "he touched my privates".

NSW police were notified and the victim was interviewed by investigators. During the interview, she alluded to further offences on previous occasions.

The accused was arrested and the matters are now before the court.

The case will go to the district court for sentence in June.