The three-year-old, who also won this year's Irish Derby and the Juddmonte Stakes at York, had been expected to run at British Champions day at Ascot next Saturday but his owners took the surprise decision to bring the curtain down on his career.
"He developed a problem in a right hind hoof. The decision was made yesterday to retire him," a representative from the Coolmore stud farm told Channel Four television.
Australia won five of his eight races, earning over two million pounds in prize money, and was described by Aidan O'Brien, who has trained five Derby winners, as the best horse he had ever worked with.
(Reporting by Sam Holden, editing by Tony Jimenez)
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