The 18-month-old boy was sweating and screaming when a member of the public noticed him locked in a four-wheel drive at McGraths Hill, in Sydney's north-west, about 11am on Tuesday.

He smashed a window of the vehicle to rescue the distressed toddler after attempts to alert the parents via the public address system of a nearby shop failed.

The temperature inside the vehicle had soared to an estimated 60 degrees during the 20 minutes the boy was locked inside.

Police charged the toddler's 34-year-old mother, who has four other children, with leaving a child unsupervised in a motor vehicle. She is due in Windsor Local Court on January 27.

A woman known as Irene, who said she was a friend of the mother, said what happened was a complete accident.

''For some reason in the Christmas havoc, she totally forgot he was in the car,'' Irene said.

''She's so upset and distressed about what has happened.

''This poor little baby is everything to her, and she would never deliberately do it.''

Irene said the mother had just dropped off her other children and forgot the boy was still in the back of the car.

''It was a terrible accident that she's never going to forget, but she is the best mother I know,'' she said.

The boy's mother has been contacted by welfare caseworkers investigating the matter.

Community Services Minister Linda Burney said, ''I understand that this incident is out of character for the mother and she is very remorseful.

''She knows that she's made a very stupid mistake.''

Ms Burney said leaving a child locked in a car in any circumstances was against the law and parents could face fines of up to $22,000.

Real estate agent Peter Cassidy, himself a grandfather, used a hammer to smash the Toyota's window on Tuesday.

He rescued the boy with help from nearby shop manager Colin Grinyer.

''When I first looked in there the kid, the baby, was very distressed, very distressed. There was perspiration pouring off him and he was very red in the face,'' Mr Cassidy said.

Mr Grinyer said, ''As soon as I picked him up out of the seat and put my arms around behind the baby's head that was just soaking wet ... the seat as well.''

The boy was taken to hospital as a precautionary measure.

AAP