A Sydney man who tortured and then murdered his niece before dumping her body in a blowhole has been sentenced to 46 years in jail.

In April 2016, Derek Barrett bound and gagged 25-year-old Mengmei Leng in their home at Campsie in Sydney's south-west.

Barrett, 29, took naked photos of the Chinese University student, before stabbing her more than 30 times and later dumping her body in a blowhole on the Central Coast.

During sentencing, NSW Supreme Court Justice Helen Wilson described the crime as a "depraved and sadistic act by a man she trusted".

"What can be said is that during the during the period of detention, Ms Leng was subjected to conduct that terrified and degraded her," she said.

"She must have been in great pain and fear."

The family of Mengmei Leng leave court after Derek Barrett's sentencing. ( ABC News: Jamie Toomey )

Barrett told police, psychologists and the court wildly differing accounts of his past drug use, but claimed to have been so high on ice and synthetic cannabis in April 2016 that he had no memory of the murder.

Justice Wilson dismissed this claim saying instead it was a crime of "extreme violence and brutality, humiliating and terrifying" and that it stemmed from his "perverted sexual obsession with the victim".

The court was told Barrett was a "sexual deviant" who had also pleaded guilty to other offences conducted in the lead-up to the killing, including secretly recording the victim and her cousin showering, and masturbating over them as they slept.

Justice Helen Wilson said these offences were grave breaches of trust as "both women were vulnerable in their sleep, and vulnerable to this breach of trust from the offender".

Barrett sat with his head bowed during the sentencing, and made little reaction he was jailed for a minimum of 34-and-a-half years. He will first become eligible for parole in 2050.

Mengmei Leng's family were in court to see Barrett sentenced, and had called for life imprisonment. They were too distressed to speak with the media afterwards.

Homicide detective Gary Jubelin addressed the media outside court, welcoming the lengthy sentence.

"The courts have recognised the seriousness of the offence by the sentence that was handed down on Mr Barrett," he said.

"From an investigative point of view it is satisfying that we've got justice but there is no joy in a matter like this it's just an extremely sad case."