MELBOURNE, Nov 14 - The youngster was found by one of his parents in the car about 2.45pm on Monday in Maddox Road at suburban Newport when temperatures in the city had soared to 32C .
The boy was rushed to hospital where he remains in a critical condition on Tuesday.
"As you may be aware, an incident occurred today at school involving a staff member and a member of their family," Newport Gardens Primary School principal Simon McGlade said in a statement to parents and carers on Monday.
"The incident didn't involve any students from our school," he said.
Footage from the scene showed police examining a black people mover parked in the street, which was later removed.
"It is unclear ... how the boy came to be in the car, however the incident is not believed to be suspicious," police said in a statement.
Victoria's Health Minister Jill Hennessy offered a general warning to parents on Tuesday.
"Get your children out of the car, do not leave them there, whatever inconvenience that involves, it's a price worth paying," she said.
"It can be really inconvenient to get them in and out of baby seats, parents are really busy, but a child left in a car can easily die."
The school sent well wishes to the family involved.
"Support will be made available for any staff, students and families that need it," McGlade said, asking for privacy for those involved.