Sutherland police said he was walking to Bates Drive in Kareela, when two men approached him at 11pm, one assaulted him and took his mobile phone.
He suffered a broken jaw and was treated at St George Hospital.
The first man involved is described as being of Caucasian appearance, aged in his early 20s, 180cm tall with a large build.
The second man is of described as being of Middle Eastern/Mediterranean appearance, 185 to 190cm tall, with a large build.
Police said the car they used to drive from the scene was thought to be an older model white sedan, and driven by a third man. Anyone with information is asked to contact Sutherland Local Area Command or Crime Stoppers on 1800 333 000.
Source: The Leader