A SYDNEY barber was busy trimming hair on the hottest weekend of the year when he noticed a little boy crying in a sweltering car.

On Saturday afternoon, Issa Khadem saw a black Holden Astra parked on the Hume Highway in the south-west Sydney suburb of Yagoona, with the distressed child locked inside.

"As soon as we saw him, we were like, 'Oh my god, do we break the window, are we allowed to do that?' So we just rang the police straight away," Mr Khadem told 9 NEWS.

"He was crying, he had wet himself in the car, poor thing," he added.

Issa Khadem immediately called 000 after seeing a distressed child in a car.
Issa Khadem immediately called 000 after seeing a distressed child in a car. Facebook

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.

He's currently behind bars in Bankstown and has been 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.

Police were forced to smash the window.
Police were forced to smash the window. Channel 9

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.

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 rescue 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.

The boy sits with emergency services after being rescued.
The boy sits with emergency services after being rescued. Channel 9