Emma screamed for help, and she and neighbours rushed the child inside to cool him with wet towels, water, a fan ... a packet of frozen peas, and called an ambulance.
Little Hugh died in hospital four days later.
On Thursday, State Coroner judge Sara Hinchey told an inquest into Hugh's death that "there is no one to blame. It is simply a tragic accident". Emma Lane, she said, was not criminally responsible for the sad death.
She indicated she would not be referring the case to the Department of Public Prosecutions for criminal investigation.
The Mount Martha mum was juggling three children under the age of five as well as her part-time job as a radiographer.
Hugh had been inadvertently left in the car when Emma arrived home with her children at lunch time after taking them to kindergartens and swimming lessons.
After unbuckling and settling inside the house with her other two children, she returned to the car to collect wet towels from the boot, and went back inside, breaking her usual habit of taking all three together, the Coroner's Court heard.
"At this time, Emma believed she had put him in his cot," counsel assisting the coroner, Jodie Burns said.
But "it then occurred to her that Hugh may still be in the car", Ms Burns said. "She screamed out for help — she was beside herself."
Hugh suffered multiple organ injuries and had worsening cerebral swelling. His life support was switched off four days later.
'Too much on her plate'
The inquest was told the boy's death was the result of a sleep-deprived mum with too much on her plate.
"There is no evidence to suggest anything other than Emma forgot Hugh was still in the car," Ms Burns said.
She brought up the psychological phenomenon of a "false memory" — when someone believes something that never happened.
She also said a person's short-term memory can only hold so much, and if overloaded, things fall out.
She said mothers of young children were often sleep-deprived and juggling many things, and hoped other parents would take note and do all they could to prevent such tragedies.
"They are fallible and they are under extreme stress, and must be aware of this fatal distraction problem," Judge Hinchey said.
"Sadly in this case, catastrophically in this case, it had fatal consequences."
The coroner will deliver her findings at a later date.
Five children have died in Victoria in the past decade after being left in cars.
