The accused, who cannot be named, is currently on trial in the ACT Supreme Court on 13 charges relating to alleged child abuse said to have occurred in a Palmerston home in the 1990s.

He allegedly molested his partner's three children, two young girls and their brother.

The trial began on Wednesday, and Crown prosecutor Sarah McMurray told the jury that the children bore "silent witness" to each others abuse.

She said none of the trio spoke out at the time. They had been subjected to physical violence, Ms McMurray said, and allegedly watched as the accused beat their mother, including dragging her by the hair through the house.

"They were subject to his violence, and felt they could not begin to question his abuse," Ms McMurray said on Wednesday.

In one incident, it is alleged the sisters were made to touch the man at the same time on his bed.

The eldest of the two sisters was allegedly told to kiss him on the mouth as "practice for when she was older".

In another incident, one of the girls was allegedly beaten with a belt after the man found jeans in her schoolbag. She was taking the jeans so she could get changed out of the tight shorts her alleged abuser wanted her to wear, the Crown says.

But the man, represented by barrister Richard Thomas, has denied the truth of the allegations.

Mr Thomas used his opening on Thursday to urge the jury to avoid deciding the matter on innuendo or suspicion, and reminded them of the importance of the presumption of innocence.

"Your role, your very important role, is to decide this matter on the basis of the evidence," he said.

The brother, now an adult, began giving evidence via videolink on Thursday.

Under cross-examination, Mr Thomas put it to the witness that he was wrong about seeing his sister on the bed with the accused during one of the alleged incidents.

He accused the witness of being concerned about losing an inheritance tied to his mother's property, and put it to him that his mother had tried to get the accused to invest in her business.

Mr Thomas then put it to the man that he had discussed his evidence with his sisters and mother before the trial.

The responses to the questions became aggressive, and appeared to reach boiling point when Mr Thomas accused the witness of being "clearly wrong" on aspects of his evidence about his sister and the accused.

The witness said:

"What the f--- would you know, you f---ing maggot."

He then walked out of the remote room, and the cross-examination was not able to continue.

Acting Justice Stephen Walmsley allowed the jury to leave for the day. The trial is expected to resume on Monday.