Emergency services were called to the suburb of Chester Hill in Sydney's west at around 3.30pm and found the small boy unconscious inside a locked car, news.com.au reports.
Parademics tried to revive the child, but he was pronounced dead at the scene.
News.com.au reports that the boy was a twin, and his mother had left him in the care of other family members for a short period of time.
Police were setting up the crime scene when the mother returned home, upon hearing the tragic news she broke down in tears and was comforted by neighbours.
According to news.com.au, it is understood that the boy was in the care of his grandmother when he was accidentally left in the car.
The woman, who was believed to be the boy's grandmother, was seen getting wheeled out on a stretcher and taken to the hospital to be treated for shock.
A spokesperson for the NSW police has said: "The child had been left unattended, we're still looking into the circumstances surrounding his death."