Prosecutors say the teenage girl became ''infatuated'' with her 35-year-old foster father, who cannot be named, telling friends she was in love with him.

The pair allegedly began a sexual relationship in 2010 after the girl came home upset over an argument with other students at her school.

The Crown alleges the foster father began to comfort her, then kissed her on the lips.

The girl was said to have recoiled, but the man allegedly kissed her again before taking off her clothes and performing oral sex on her. Prosecutor Anthony Williamson said the man then tried to have sex with her.

The Crown said that was the start of regular sexual encounters between 2010 and 2011, when the girl was 14 and 15.

In an interview with police, the girl said she was having sex with the man on a daily basis. She said her foster mother was always out at the time.

''When it first started, it was almost every day, and that's why I was always really late to school,'' she told police. ''A lot of the time it was after school as well.

''And then it stopped being so much, and it would be once a month.''

The foster father, who is facing 12 charges, went on trial before a jury in the ACT Supreme Court on Friday.

The man is fighting the allegations and his barrister, James Sabharwal, told the jury they must take great care when listening to the evidence.

Mr Sabharwal asked the jury members to put their views about the nature of the allegations to one side.

''This is not the place for you to express those views,'' he said. ''Your duty in this case is to assess the evidence, and do so fairly.''

The Crown says the carer told the girl that if anyone found out about their relationship, she should ''deny, deny, deny''.

It was alleged that the man emotionally blackmailed the girl, telling her he would kill himself if she told anyone.

''We say the accused felt sexual urges towards [name removed], who was at the time an attractive teenage girl,'' Mr Williamson told the jury.

He said the defence was likely to argue that the sex never happened, and that the girl was making it up. But he said the girl's story had remained consistent since the allegations were first made.

''How hard would it be to maintain this lie, if it were a lie?''

The Crown said the girl originally tried to protect her foster father, telling police that nothing had happened.

The trial, which the jurors were warned was not going to be ''particularly pleasant'', continues on Tuesday before Justice Richard Refshauge.