Cecil Patrick Kennedy is on trial over the death of toddler Jordan Thompson in Singleton in 2005

As she approached the unit block where she'd left her toddler in the care of her boyfriend, manslaughter-accused Cecil Patrick Kennedy, she heard the word 'help'.

"I looked up and realised it was coming from Cecil's unit, and it was his voice," Ms Swales said.

She remembers her boyfriend's son appearing at the top of the stairs, saying "Quick, there's something wrong with Jordan".

Her son was lying naked on the bed, and Mr Kennedy was breathing into his mouth. He was pronounced dead a short time later at Singleton Hospital.

Ms Swales was the first witness to give evidence in the trial against Mr Kennedy, 51, who has pleaded not guilty to the alleged unlawful killing of baby Jordan.

Her evidence followed an opening address from Crown Prosecutor Kate Nightingale who said that, contrary to what he told police, Mr Kennedy left his unit to use a public phone booth that afternoon that Jordan was left in his care and died, March 19, 2005.

He made two calls to a "close associate" who will give evidence to say that during that conversation, Mr Kennedy talked about giving a child sleeping tablets to sleep, the court heard.

The woman would also allege Mr Kennedy got anxious and stressed when children, including baby Jordan, cried, the court heard.

Jordan had been with his mum and an older sister at Mr Kennedy's unit in Singleton that day, the jury was told. Just before 5pm the Ms Swales left the unit, and returned an hour or so later.

Other elements of the crown's case includes that traces of anti-depressants were found in a blood sample later taken from the toddler, medication which police later found in Mr Kennedy's wardrobe and which were prescribed for him, Ms Nightingale said.

The trial, expected to run for an estimated eight weeks, continues.