The mother had dropped the boy and his siblings off at St Margaret Mary's Catholic Primary School in Melbourne yesterday morning, police said.
However, she did not realise that her eight-year-old son had snuck into the back of the van and hid.
She returned to her vehicle and found him inside at 2.45pm.
Temperatures inside a closed auto can reach nearly double the air temperature outside.
A child has been taken to hospital suffering from critical injuries after being trapped in a vehicle in hot conditions in Melbourne's west, police say.
'It is unclear at this time how the boy came to be in the auto however the incident is not believed to be suspicious at this time, ' Sen-Constable Thomson said.
The boy was rushed to hospital wheere he remains in a serious condition. "Police are treating the incident as an accident".
The principal of Newport Gardens Primary School where the woman worked released a statement to parents and carers on Monday.
The boy's school is less than four kilometres from where he was found on Maddox Rd, Newport.
His classmates are also struggling to cope with the tragedy, and both the boy's school and the public school where his mother works are offering their communities counselling.
Police, paramedics and fire brigade crews had all rushed to the scene, which is close to a primary school and childcare centre.
Police told the Herald Sun they did not need to break into the auto to get access to the child.
Nearby residents told the Herald Sun they did not recognise the Kia or know whether the boy's family lived in the street.
Another resident who saw the commotion said it was a 'very sad situation'.
"Get your children out of the auto, do not leave them there, whatever inconvenience that involves, it's a price worth paying", she said. 'It's sad whichever way it happened - especially when kids are involved'.
Police said they are treating the incident as an accident.
The city has sweltered through a brutally hot spring day with the mercury hitting 33 degrees.