When they arrived they saw a distressed boy, possibly aged between two and three, locked in a Kia Carnival. They then broke the window to access and remove the child.
The boy was checked by NSW Ambulance paramedics and was reported to have mild dehydration. He was sent to the Royal North Shore Hospital.
Police said the child was alone in the car for three hours.
The Telegraph reported police left a note on the front door of a nearby home saying, "Call North Sydney police as your car has been impounded as you left your child in the car."
The boy's stressed father arrived back on the street to find his car — and child — gone.
"We didn't sleep through the night. It was a rough night and I went to drop off my kid at school," he told reporters.
"He fell asleep in the car and I thought he was at school and I went straight to work. I was very confused."
With no car, the father had to hitch a ride with reporters to the hospital to see his kid. Police have spoken to the child's parents.
The force said the incident highlighted the dangers of leaving kids alone in vehicles.
"There is a risk of being abducted if they are left alone in a car, even if the car is locked," police said in a statement.
"On a typical summer day, the temperature inside a car (even with the windows rolled down a little) can quickly rise above 50 to 60C.
"Even on a relatively mild day, the temperature inside a car can get above 40C".
At those temperatures, children would be at risk of heat stroke, which can lead to a high fever, dehydration, seizures, stroke and death.
