The man, who cannot be named, was sentenced in the ACT Magistrates Court over two incidents at his family home in 2011.

The victim was at home with the offender's daughter, also 16, one evening in late March.

While they were out of sight, the offender put his hands down the pants of the teenager, grabbing her buttocks.

A short time later, the offender, who had been drinking, pulled the girl into a hallway and again put his hands down her pants, grabbing her buttocks and telling her she was "beautiful".

The teenager pushed the older man away, and he reached out and grabbed her breast.

He was found guilty on two charges of act of indecency without consent by Magistrate Peter Morrison.

The 56-year-old was sentenced to a two year good behaviour order in the ACT Magistrates Court on Thursday morning.

Magistrate Morrison said the defendant was "very close" to being sent to jail, and had "avoided the full consequences" of his actions by agreeing to the good behaviour order.

The court heard earlier that the indecent assault had devastated the victim and her mother.

The victim suffered anxiety, felt guilt, shame, desperation, and begun avoiding her friends and all social interactions, according to a victim impact statement.

She put on weight, had trouble sleeping, and went through a "significant period of feeling worthless".

She begun locking doors around the house at night, and her grades suffered at school.

Her mother also suffered from lasting effects of the incident, the court heard.

She said in a victim impact statement that the crime had caused a "major upheaval" in their lives, forcing them to move interstate.

The mother said she felt she was unable to protect her child, and felt as though she was powerless to stop the crime.