Michelle Nguyen leaves court after an earlier hearing.

Michelle Nguyen leaves court after an earlier hearing. Photo: Craig Abraham

A mother who has pleaded guilty to her 10-month-old daughter's manslaughter by leaving her in a car on a hot day was repeatedly warned about the dangers of doing so, a court has heard.

Michelle Nguyen's daughter Thy Tran died on November 5, 2011, after the-then 20-year-old mother left her asleep in the car while it was parked in direct sunlight and with the doors and windows closed.

The temperature on the day the girl died was 26.3 degrees and, at 3pm, was 28.2 degrees.

Nguyen had taken Thy and her other daughter Thao, 3, for a drive to get them to sleep.

After about 30 minutes, when they returned to the Glenroy home where they were staying, Nguyen removed Thao, who had woken, but left Thy sleeping in the car in the driveway.

After settling the older child inside the house, Nguyen returned to the car five minutes later to check Thy. The infant was still asleep, so she closed the car door and then went inside, intending to return soon after.

But while inside, she fell asleep in a chair.

The court heard Nguyen woke in a panic about 2pm, realising that her daughter was still in the vehicle. When she removed her from the car, the child was unresponsive.

She brought her inside and called an ambulance, but the child could not be revived.

Thy's temperature was tested by paramedics and found to be 41.5 degrees.

At Nguyen's pre-sentence plea hearing in the Victorian Supreme Court on Friday, prosecutor Mark Rochford, SC, said Nguyen had repeatedly been warned of the dangers of leaving her children in the car before her daughter's death.

Relatives and neighbours had on numerous occasions overheard Thy crying unattended in the car and, on one occasion, Nguyen was also told about another child who had died after being left alone in a car on a hot day.

Mr Rochford said Nguyen had initially given a false account of what happened to police, stating the child must have suffocated after rolling on to her stomach on a mattress.

She later admitted what had happened and that she was responsible for her daughter's death.

Nguyen wept in the dock as details of her crime were read to the court.

Her defence barrister, Saul Holt, said his client was deeply remorseful for her actions.

He submitted that Nguyen was going through a tumultuous period at the time it happened as she was homeless with two small children whom she was struggling to care for while their father was in jail for drug offences.

He said Nguyen had a difficult upbringing, largely due to the fact her mother was a drug addict who was in and out of jail.

Nguyen had herself developed a heroin addiction from the age of 16, suffered from severe depression and, according to one psychiatrist, had shown signs of post-traumatic stress disorder after her daughter's death, he submitted.

She had also lost custody of Thao following Thy's death, but was taking positive steps to get her life in order and overcome her drug habit.

He added that his client would regret her actions for the rest of her life and noted that despite her tendency to leave the children in the car sleeping, she was otherwise a good mother.

He called for a sentence to be served through a community based order.

But Mr Rochford said a higher sentence was warranted to send a strong message to the community that children should never be left in cars.

He said Ambulance Victoria had stated that more than 900 calls were received each year regarding children left in vehicles despite repeated warnings about the dangers.

He did not, however, rule out a shorter than normal non-parole period being imposed.

Justice Betty King will sentence Nguyen at a later date.