Mother of five Kate Leithhead said it was a three-minute walk to her drug dealer's apartment in Richmond where she would score heroin but she had never had sex with him.

Ms Leithhead agreed with defence barrister Saul Holt, SC, that her former partner, Paul Ta Vuong, had never accused her of sleeping with her drug dealer.

Mr Ta Vuong has pleaded not guilty to murdering Ms Leithhead's son, Silas, on May 15, 2013, after she went to buy heroin.

Cross-examined by Mr Holt, Ms Leithhead agreed Richmond was her "town" because Mr Ta Vuong lived in Burwood with his parents so it was her job to buy the heroin because she had connections to easily score the drugs.

Asked if she was sleeping with the drug dealer, Ms Leithhead said: "Why would you ... yeah, no."

Crown prosecutor Andrew Grant has told the jury Mr Ta Vuong allegedly boasted to two prisoners when he was on remand how he had dropped Silas on his head five times because he was angy at having to wait too long for Ms Leithhead to get heroin from the drug dealer.

Mr Ta Vuong allegedly believed Ms Leithhead, was "f...ing round with" or "f...ing" the drug dealer so he decided to play rough with Silas.

Mr Grant said Mr Ta-Vuong picked up some bedding in his cell and used it to demonstrate how he threw Silas to the ground.

Mr Holt told the jury the two prisoners were liars and Mr Ta Vuong had never intentionally meant to hurt Silas.

Mr Ta Vuong claimed he was carrying Silas from the bathroom when he slipped and lost control of the boy who fell and hit his head. He believed he might have fallen on top of Silas. Silas died from blunt force trauma to the head.

Ms Leithhead said her relationship with Mr Ta Vuong started to go downhill after they began using heroin again on a daily basis in February or early March last year. They began to argue about drugs and sharing drugs.

She and Mr Ta Vuong had met in late September 2012 when residents at Odyssey House, a rehabilitation facility for people suffering from drug and alcohol addiction.

Ms Leithhead was five months pregnant with Silas at the time.

Ms Leithhead said they had both wanted to get off drugs so they could have a future together and Silas would not grow up in a drug environment but they continued to use heroin.

Ms Leithhead would usually drop Silas off at a childcare centre in Richmond every morning between 8.30am-9am and spend the day at home using drugs if she had some before collecting him at between 5.30pm-6pm after watching The Bold and the Beautiful.

But on the day Silas died he was sick so she kept him at home before leaving him in Mr Ta Vuong's care while she went to buy heroin.

The trial, before Justice Karin Emerton, continues.