Dennis Michael Nona, 41, has pleaded not guilty in the ACT Supreme Court to two counts of having sex with a child and two counts of an act of indecency in the presence of children.

In her opening statement, Crown prosecutor Margaret Jones told Chief Justice Helen Murrell and a jury that Nona had sex with the girl in her home in February 1996 and again in April, causing the girl to fall pregnant.

But Nona told the jury he had never had sex with the girl he met while visiting a house known for hosting two-to-three day parties with heavy drinking.

He told the jury after two days of drinking and not eating, he began to vomit and eventually passed out in a bedroom inside the girl's house.

After waking on Sunday morning, he said the girl approached him while they were alone and said, "I want to tell you something but you're not allowed to tell mum - I had sex with you last night."

Nona told the court his "body turned cold" and he wanted to leave the house immediately and asked her, "why did you do that you're going to get me into big trouble".

Nona said it was common for parties at the house to last for three days with drinking from morning until night, and that many men would drink until they passed out on the floor.

In his opening submission, Nona's barrister John Purnell told the court the girl had "a crush on [Nona] and was keen" on the accused, despite admitting the circumstances were shocking and unusual.

But in a formal police interview in March 1997, the girl said she didn't know how she became pregnant and didn't remember having sex with anyone.

She told police she remembered waking up in her bed after a party where many men had been drinking and being told she had had an epileptic fit, a condition she was diagnosed with in 1995.

The girl said she had approached her teacher to ask how a woman became pregnant and what the symptoms were, and began to cry when she recognised she had had morning sickness and could feel something moving in her stomach.

Her pregnancy was not discovered until police were called to the house for an unrelated matter and her pregnancy was terminated 22 weeks after conception.

Nona, who is from Badu Island roughly 60km north of Thursday Island, is also accused of committing an act of indecency in front of two children aged 13 and 14 at the time.

The incidents are said to have happened while Nona was studying fine art at the Australian National University.

In Ms Jones' opening submission, she told the court the two children were in the backyard of a property when Nona knocked on a bedroom window from to draw their attention, and then exposed his penis with his hand on it.

The complainant told the court on Tuesday Nona was allegedly "playing with his penis as if it were a joke" and was "looking right at us".

Nona denied ever doing so. The trial continues in the ACT Supreme Court.