Police arrived to smash the window and immediately treated the boy for dehydration.
The father was later found drinking at a nearby pub, the Hume Hotel, and was arrested.
The father spent time behind bars in Bankstown and was charged with leaving a child in a motor vehicle causing distress, neglecting a child in his care and resisting arrest.
When the boy's mother arrived to pick her son up, police discovered the father had left his other son, a two-year-old boy, alone in his Yagoona home.
Sydney sweltered through its hottest September night on record on Saturday with temperatures struggling to drop below 25C even at midnight.
Saturday, the day the boy was left in the car, was Sydney's first ever 40C September day.
The father appeared in Parramatta Local Court Monday where he was denied bail.
As we head into the hotter months, emergency services have a stark warning about leaving children in hot cars.
Police continue to remind parents that on a typical summer day, the temperature inside a car, even with the windows rolled down a little, can quickly rise above 50C, news.com.au reports.
Even on a relatively mild day the temperature inside a car can get above 40C and, at those temperatures, children are at great risk for heat stroke, which can lead to a high fever, dehydration, seizures, stroke and death.
With the warmer months approaching, parents should be even more aware.
On one January day this year, emergency workers in Victoria had to reduce a whopping 13 kids from hot cars
At the time, State Control Centre spokesman Graeme Baxter called the number "disappointing".
Children should not even be left in cars parked undercover, Mr Baxter said.
"It's just not a safe thing to do, no matter where you are. We really shouldn't be leaving kids alone locked in cars. We should be taking them with us or finding someone else to take care of them. A car is not a place for childcare," he added.
