Jian Chen, 47, was originally charged with murder but the NSW Supreme Court accepted her guilty plea to the less serious offence of manslaughter.
According to a statement of facts before the court, reported by Australian media, police said the man, Xian Peng, 48, returned to Australia from China in February 2011 with his new girlfriend.
Police said that days later, Jian used pills to spike the soup she gave him at her home and when he was asleep, tied his hands and feet and stabbed him a number of times in the neck and groin, before hacking at his penis.
"An attempt to castrate him had taken place," said the statement of facts. "The victim also had lacerations to the left side of his groin and his penis had been scalped." She was remanded in custody until sentencing in August.