Olympian Ian Thorpe has checked himself into rehab in Sydney while battling depression and alcohol abuse.

The swimming great has been living in Switzerland for 18 months, but returned to Sydney to spend the Christmas period with his family at his parents' Panania home.

The 31-year-old was injured in a fall earlier in the week, according to media reports on Friday, and was admitted to hospital on Wednesday night.

Friends say Thorpe has struggled to adjust to life after swimming and with the disappointment of a failed comeback at the London Olympics in 2012.