A man who murdered his niece and dumped her body in a Central Coast blowhole says he has no recollection of his crime because he was on a drug binge that had caused him to black out.

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.