An eight-year-old boy found unresponsive in a car in Melbourne's west in scorching temperatures had climbed into the back of the vehicle and hid during the school morning drop off without his mother knowing, police say.

The boy's condition on Tuesday improved from critical to serious after he was discovered by his mother inside the family's black Kia minivan on Maddox Road in Newport about 2.45pm Monday as the temperature hovered around 32 degrees.

Emergency services were called to the scene just before 3pm. The boy was reportedly in cardiac arrest and not breathing.

The boy's mother dropped her children at St Margaret Mary's Primary School in Spotswood in the morning without realising her eight-year-old son had climbed back into the car and hid himself.