The little boy was reportedly left inside a car outside a childcare centre in Kyneton, Australia.
"We're devastated"
Local media outlets have said that he was discovered by his mother when she went to pick him up from the centre, although police have been unable to confirm these reports.
"That's all part of our inquiries at this stage," Inspector Ryan Irwin told The Herald Sun.
"I understand the boy has been found by a family member and paramedics were called and obviously there were attempts made to try and revive the child."
The police officer, who added that homicide squad detectives will be overseeing the investigation, called the incident an "absolute tragedy".
He said emotionally: "We're devastated.
"[The family are] devastated, absolutely devastated and we're still trying to track some family members down to actually notify them."
Temperatures reached 31.6C in the Kyneton area, meaning the car's interior would have reached almost unbearable temperatures.
Ambulance Victoria group manager Brett Drummond has said it is frustrating that people have still not learned how dangerous it is to leave children in cars.
He said: “It’s never okay to leave a child in a car, no exceptions, no excuses.”
Our thoughts are with the little boy's family.
Pictures via 9News.com