She believed the dismembered and decomposing body was her son, Khanh Xuan Pham, a regular ice user who had repeatedly told her someone wanted to kill him.

She would later learn the body was that of Goran Stefanovic, and that her son had stabbed him to death.

Pham, 40, pleaded not guilty on Tuesday to the murder of Mr Stefanovic in January 2019 in a NSW Supreme Court trial.

Pham admits killing Mr Stefanovic, but the Crown did not accept his plea of guilty to manslaughter.

Thi Hon Pham made the gruesome discovery in her son's bedsit in Sadleir in southwest Sydney on January 12, 2019.

Through a Vietnamese interpreter, she told the court she usually visited her son on Saturdays, bringing him food and offering to clean his untidy apartment.

In the weeks leading up to the killing, she noticed he was changing and becoming paranoid, she said.

He told her he was dying of brain cancer that had spread to his legs and feet. She did not believe it, blaming his drug use.

He also said someone wanted to kill him, and separately claimed he feared a neighbour would come through his kitchen wall to murder him.

Another time, Pham dragged his mother into the apartment quickly when she arrived, saying "Mum, come in really quickly before somebody kills you".

Pham was not home when his mother came by on January 12, so she let herself in and tidied up.

A large number of flies were in the apartment, she said.

As she readied to leave, she noticed a growing bad smell, and realised she had not cleaned the bathroom.

"I started getting goosebumps on my arms," she said.

She poked her head into the bathroom door and saw two legs.

"The only thing I could think of at the time was that it was my own son," she said. She only realised it wasn't him when he called her later that day, after police had already arrived at the apartment.

The parties agree that Pham and Mr Stefanovic had known each other for a few weeks at the time of the killing on January 8.

Mr Stefanovic came to Pham's house that day after messaging him to organise a purchase of ice.

Mr Stefanovic had been talking on the phone before Pham picked up a kitchen knife and stabbed him in the back.

He had not done or said anything threatening to Pham, the parties agree.

The victim yelled "what have you done" before running out the front door. Pham followed him then stabbed him in the heart, chest and back.

At some point, Pham dismembered the body and left it in his unit's shower.

The defence will argue Pham was affected by drugs and thought Mr Stefanovic was trying to kill him.

The court was shown a police interview with Pham after he was arrested on January 13.

"I'm good at reading people, reading their body language ... It's hard to lie to me," Pham told investigators.

"He was going to kill me, so I killed him."

Pham's girlfriend, Kate Lloyd, is in prison after pleading guilty last May to accessory after the fact to murder.

She helped Pham drag the body back into the apartment, then used Mr Stefanovic's car with Pham until the two were arrested.

The trial continues on Wednesday.

Australian Associated Press