A young child has been found dead, locked in a hot car in south-west Sydney.

Emergency services rushed to Railway Parade in Glenfield following reports a child was not responding after 3pm on Thursday.

Paramedics attended however the three-year-old, believed to be a boy, was not able to be revived.

Police arriving at the scene were told the child had been in the car throughout the day, with temperatures remaining at more than 30 degrees Celsius throughout the afternoon.

The owner of the vehicle raised the alarm when he returned to the car.

The man, who is known to the child, was taken to Campbelltown Police Station where he is assisting police.

Police are investigating and have established a crime scene.

More to come