Police said a member of the public called them to a unit on William Street, Bankstown, about 5.40pm on July 4 to check on the man.
But they didn’t show up until 12.30pm the following day.
Officers started CPR but paramedics arrived and declared him dead.
Police said a crime scene was established and today a critical incident was declared.
“The Critical Incident Investigation Team from Cumberland Police Area Command will now investigate all circumstances surrounding the incident, including all aspects of the response from NSW Police,” NSW Police said, in a statement.
“That investigation will be subject to an independent review by the Professional Standards Command and oversighted by the Law Enforcement Conduct Commission.
“A report will be prepared for the information of the coroner.”
The LECC confirmed it was notified of the critical incident today and would be “independently monitoring the investigation”.