Sydney man who sexually abused his stepdaughter as she slept says it became a "game".

Warning: The following post contains details some may find triggering.

A Sydney man who filmed himself sexually assaulting his young stepdaughter as she slept says his years-long “relationship” with the girl wouldn’t have happened if she was his biological child.

The man, who can’t be named for legal reasons, filmed himself committing sex acts on the sleeping girl and says it became a “game” to see how much he could do to her before she woke up.

He told the Downing Centre District Court the girl, who he abused from the age of 12, was treated differently to other children in the family.

“Biology is one thing. There’s no attraction to your own children,” the self-represented 45-year-old, who had known the girl since she was four, told the court on Wednesday.

“I think my reaction would have been different if it had been my own biological child.”

The man has admitted to 99 offences relating to the girl’s sexual abuse from 2009 to 2014 but is on trial after pleading not guilty to drugging her and her mother.

He also denies assaulting a younger stepdaughter, saying he was present at her birth and felt like her biological father.

The court has previously heard SD memory cards discovered in the lining of the man’s jacket contained 78 videos and 778 images showing him sexually and indecently assaulting the older stepdaughter.

The girl gave evidence in a closed court but parts of her testimony have been read out at the trial.

The court heard she gave evidence about not being “completely awake” on one occasion as the stepfather had her bent over her bed.

“I remember I couldn’t wake up,” she said.

The crown alleges the stepfather drugged her mother so that he could have greater access to the girl but the man says it was the girl who was drugging her own mother.

The court heard the mother gave evidence about often feeling exhausted and seeing white flecks in her drinks.

The crown also alleges the girl, who was at one point assaulted with a plastic object while asleep, appears to be snoring in parts of the footage.

“Sexual gratification was achieved but no intoxicating substance was administered,” the stepfather said.

The trial continues.

If you or someone you know is dealing with domestic or sexual assault, please seek help from 1800 RESPECT. If you or your family is in immediate danger, please call 000.