Australia's Daily Telegraph reports that audience members saw the 64-year-old fall face first off the stage and hit his head.
Buffett's manager then took the stage and urged fans to leave the auditorium, the Hordem Pavillion.
The singer was taken to Sydney's St. Vincent's Hospital.
The concert Wednesday was Buffett's fifth and last in Sydney on his current tour.
TMZ reported that the entertainer lay unconscious for about five to 10 minutes. He was reported in stable condition and will remain at the hospital for observation.
As for why the incident happened, eyewitness accounts vary. TMZ reported that one fan who saw the spill said flashing stage lights during Buffett's solo may have temporarily blinded him. Another fan said it appeared the entertainer was trying to leap off stage to shake hands with audience members and may have simply mistimed the jump, hitting his head on a metal bar.
TMZ contributed to this report.