Police were called to the condo at 7:30pm after staff discovered the body of the 27-year-old man lying on the floor of the room.
Karon Police investigating officer Pol Lt Col Eaksak Khwanwan reported the incident to Karon Police Chief Pol Col Pathapi Srichai before officers, forensic personnel and rescue workers attended the scene.
According to police, condominium staff said they last saw the tourist at about 12:20am on Tuesday (May 19). Later, concerned after being unable to contact him, staff alerted the room owner, who entered the room and found the man unresponsive before notifying police.
Investigators reported finding a quantity of white powder in a dish on a table near the body. Initial examinations found no signs of a struggle or physical assault.
Forensic officers collected evidence from the room, while the body was transferred to Vachira Phuket Hospital for a post-mortem examination to determine the exact cause of death.
The substance found at the scene will also undergo forensic testing as part of the investigation.
Police said the Australian Embassy has been notified.