The devastating find was made on a blue Goodstart Early Learning bus in the carpark outside Hambledon State School in the southern suburb of Edmonton on Tuesday.
“The boy was located deceased on a minibus parked out the front of a school on Stokes Street at 3.15pm,” Queensland Police said in a statement on Tuesday night.
“Cairns detectives from the Child Protection Investigation Unit are investigating the circumstances surrounding the incident.”
The temperature reached 34C yesterday in Cairns.
A police spokesman told news.com.au the male driver of the bus, who discovered the young child, was taken to Cairns Base Hospital “suffering shock and distress”.
At the scene yesterday, Detective Inspector Jason Smith said it was “early days” and “a number of factors could be at play here”.
Nine reported the boy boarded the bus this morning and was supposed to be dropped off at a local daycare centre but remained on the bus for all of Tuesday.
According to the ABC, police said he had been picked up from his house yesterday morning but would not confirm that he had been left in the vehicle throughout the day.
“We’re just trying to work out exactly what has happened between now and when he should have been delivered to a daycare centre,” Insp Smith said.
“The death of any child is an awful thing which is why it’s so important for us to get to the bottom of this.”
The boy’s mother was notified.
In a statement to news.com.au on Tuesday night, a spokeswoman for Goodstart Early Learning said the company “is devastated by the death of a child in our care in Cairns earlier today”.
“We are working to support the entire centre team, our families and our children,” she said.
“The matter is now a police investigation and we will be working with the police, Department of Education (childhood education and care), and Workplace Health and Safety.
“Our thoughts are very much with the family of the child at this time.”
It has more than half a dozen childcare centres in the Cairns area around the primary school.
The closest weather observation station to Edmonton, Cairns Racecourse, recorded a maximum temperature of 33.8C on Tuesday afternoon. It was 34.6C at 2.25pm in Cairns city.
A report will be prepared for the coroner.