The abuse of two sisters began when the man, now 41, started seeing their mother.

He moved into the family home, and helped to look after the children, according to court documents.

The stepfather later told police he found the young female children attractive, and would binge drink every week, blacking out and becoming "horny".

On one occasion, the mother was away caring for a sick relative. The stepfather came into the younger girl's room and touched her indecently. She was eight years old.

When she was nine, he entered her room while she was watching television, pulling down her pants and touching her.

Police say he only stopped when he became bored.

The child would later estimate she had been molested more than 100 times between 2008 and 2012.

The abuse was so frequent, she said, that she hung a wind chime at her door to wake her up when he approached at night.

The girl didn't tell anyone because she said she was scared the family would not stay together.

Her abuse stopped in 2012, according to court documents. She had told him she didn't want to do anything with him, and just to forget about what had happened.

He told the child he doubted she would be able to forget, but warned he could go to jail if it was made public.

In 2013 and 2014, the stepfather also committed crimes against another girl, then 14 and 15.

She later told her mother of the abuse, prompting her to confront her partner.

Police were called and the man made admissions, telling them he was "sick in the head" and needed help.

He said he knew it was illegal and would have a great impact on the girls, but couldn't stop himself.

Police charged the man and brought him before the courts.

He appeared in the ACT Magistrates Court on Tuesday, and pleaded guilty to eight acts of indecency.

Another three charges are likely to be dropped by prosecutors after he is sentenced.

The man, who is currently in custody, was committed for sentence in the ACT Supreme Court.

He will appear in the higher court on 11 June.