A Chinese woman residing in the New South Wales has admitted before the state Supreme Court that she had indeed castrated her former boyfriend last year, in an apparent pit of rage following their breakup.

But Jian Chen told the court in Sydney that there was premeditated plan to kill 48-year-old Xian Peng, who died February last year when the accused, using a knife, sliced off the victim's penis and scrotum while he was sleeping.

Prior to the attack, Mr Peng had ended his relationship with Ms Chen, which police investigators said could have sparked the whole unfortunate episode.

According to the Australian Associated Press (AAP), pleaded guilty to the charge of manslaughter, which carries lesser penalty to the original charge of murder.

The court accepted the plea, according to ABC.

Court records showed that Ms Chen, also 48-year-old, was apparently scorned when Mr Peng got back from China with a new partner last year.

Police said that the suspect had laced the soup that Mr Peng was taking on February 9 with sleeping pills, inducing the victim into a helpless state soon after.

Investigators had established that Ms Chen tied up the victim and then started attacking him with a knife, hitting Mr Peng a number of times in the neck and the groin.

The victim's private parts were then cut off as the police statement filed before the NSW court showed that "an attempt to castrate him had taken place."

The police report also indicated that Mr Peng suffered "lacerations to the left side of his groin and his penis had been scalped."

Court records also showed that Ms Chen had alerted authorities when she summoned for an ambulance to her North Ryde residence, where medical personnel found the bleeding victim.

Police said that Ms Chen had informed emergency response team that she stabbed her boyfriend, later identified as Mr Peng who died of his injuries in a hospital.

Neighbours of the suspect, according to ABC, expressed shock on the incident and described Ms Chen as quiet and reserved.

Hearing of the case will resume August this year and Ms Chen will remain behind bars until the case has been wrapped up by the courts.