The cyclist had been riding on a track near the Sturt River in Mitchell Park just before 1am this morning when he found the man.
The rider immediately called an ambulance, and despite paramedics rushing to the scene, the man was unable to be saved.
A crime scene was established and police cordoned off the area, including parts of Karu Crescent.
Forensics went over the scene from 4am as detectives initially treated the man’s death as suspicious.
Early this morning, police said the man’s death was most likely the result of a medical episode but are continuing investigations.
A report is being prepared for the coroner.
Senior Constable Kate Dawson urged anyone with further information to contact Crime Stoppers on 1800 333 000.