Derek Barrett, 29, murdered Chinese student Mengmei Leng in April 2016 at the home they shared with his wife and stepdaughter at Campsie in Sydney's west.

On Wednesday, the NSW Supreme Court began a sentencing hearing for Barrett, who pleaded guilty in August to murder and 19 counts of filming private parts without consent.

The killer spoke for the first time, claiming he did not remember the murder nor dumping Ms Leng's body because he was high on ice and synthetic cannabis, which caused a 48-hour gap in his memory.

"AlI I remember is an argument," he told the court. "I remember seeing myself in the reflection of a mirror and looking down at blood in the sink.

"It's almost like I was waking up in the bathroom ... there was a lot of blood."

During cross-examination by Crown prosecutor Margaret Cunneen, SC, Barrett recalled seeing his niece's distressed face. She was shouting at him in Chinese, and he was apologising to her.

"I don't remember the actual stabbing itself or anything leading into the stabbing," he said.

"I remember a scared face on the bed ... she was yelling something back at me. I think she may have been cursing at me. I said 'I'm so sorry.'"

Ms Leng was 25 when Barrett tied her up with tape and took numerous photographs of her naked body, including close-up photos of her genitals, then removed the restraints and stabbed her to death.

He then bundled her body in the boot of his car and drove to Snapper Point on the Central Coast, where he dumped her body in the water. Tourists found her there shortly afterwards, on April 24.

Barrett sniffled at times as he gave evidence, mumbling, taking long pauses and fiddling with a tissue in his hands. At other times he responded forcefully, when Ms Cunneen suggested he was simply pretending he could not remember.

"There's no words," he told the court. "I can never reverse the actions that I've done. I'd gladly exchange positions - I'd rather her be here than me. She had a beautiful future ahead of herself."

In a victim impact statement, which was read by a police officer, Ms Leng's mother Mei Zhang said Barrett was a "fiend" who should be jailed for life.

She said her daughter became the centre of her life after her husband died in 2008, and she sold her home in China to help fund her daughter's future.

Ms Zhang said she still could not accept her daughter was tortured and murdered, with her body "abandoned to the sea".

"Now that this child is gone, my whole life is shattered," she said. She sobbed as her statement was read out.

Forensic psychiatrist Richard Furst told the court Barrett was most likely motivated by a sexual deviance or a sadistic fantasy, and took pleasure in having "complete" and "fatal" dominance over his niece.

In his opinion, Barrett was "basically not telling the truth about his memory loss".

"It's not the kind of thing one would forget," Dr Furst said. "His actions are all purposeful, from the time of the killing to the disposal of the body."

Barrett claimed he was taking up to 15 grams of ice a week in the lead-up to the murder, which he funded by helping to deal drugs.

His estranged wife left the court shortly after he began to give evidence, later returning with a policewoman.

Justice Helen Wilson will hand down a sentence on Friday.