A Sydney man who filmed, raped, tortured, kidnapped and murdered his terrified niece before dumping her body in a blowhole says "I let my own problems spill into the family home".
"This moment in my life has kept me awake at night in tears and I still have nightmares," Derek Barrett wrote in a letter of apology to his victim's heartbroken mother.
Already in jail for at least 34 years and six months for the murder and kidnapping, the former IT worker faced a resumed sentence hearing in the NSW Supreme Court on Tuesday in relation to the filmed sex attacks.
The 32-year-old was jailed in 2017 after admitting repeatedly stabbing student Mengmei Leng, whose body was found in the blowhole at Snapper Point on the NSW Central Coast in April 2016.
He had admitted filming the 25-year-old niece of his wife, while she was bound and naked, for his sexual gratification but a judge found he deleted the film from his phone on arrest from fear of detection.
Barrett was only charged with the sex offences late last year after an elderly woman with dementia was mysteriously found holding a USB containing video of him raping the gagged and terrified woman.
He's admitted 17 counts of assault with acts of indecency and aggravated sex assault, committed shortly before he stabbed his niece at least 30 times.
A psychiatrist testified that Barrett told him "I lost everything because of a stupid weekend".
In his letter tendered to the court, Barrett wrote: "No words can begin to describe the emotional pain I have caused to you and the family. I can only imagine what you must be going through from your loss.
"Every moment of my life I wish I could go back in time and take back that day that has caused so much pain.
"I let my own problems spill into the family home and they paid dearly as a result."
He had never wished ill will on anyone, would happily trade his life if it could bring Ms Leng back and took full responsibility for his terrible actions.
"All I can do, in some small way, is to commit my life to trying to make up for what I have done in anyway possible."
Barrett didn't give evidence, leading the prosecutor to submit that Justice Helen Wilson should give little weight to the letter.
The judge noted Barrett did give evidence at his murder sentence hearing when he said he should be given life imprisonment but he then went onto unsuccessfully appeal her considerably lighter sentence.
In the mother's translated victim impact statement, Tam Mei Zhang, asked the judge to sentence "this vicious rapist, malicious torturer and cold-hearted murderer, to life imprisonment".
"In April 2016, the death of Mengmei had brought great pain to my whole family and I, a single mum trying to support her daughter who is an international student in Sydney," she said.
"My healthy mother was in such grief that she too passed away, not long after receiving the news of Mengmei's death."
Ms Zhang said she then received a call from police saying they had found the video footage.
"When the police told me the video clip showed how my Mengmei was degradingly raped, agonisingly tortured, and brutally raped, a wave of faint immediately washed over.
"How can any mother possibly accept such news? Especially when my Mengmei was such a kind, sensible, filial and decent girl! How my heart breaks!"
Barrett will be sentenced for the sex offences on March 18.