Phillip Hughes has become the youngest player to score twin centuries in a Test, guiding Australia to a massive 506-run advantage after three days of the second Test against South Africa.

Australia bowled South Africa out for 138 in the first over on Sunday and batted for the rest of the day, reaching 3-292 at stumps at Durban's Kingsmead ground.

Hughes scored an unbeaten 136 in 346 minutes, including 13 fours and two sixes, adding 164 for the second wicket with captain Ricky Ponting (81).

The skipper holed out to deep square leg off Morne Morkel's bowling, leaving Hughes on 98.

Opener Hughes cut a boundary over slips to reach his century from 247 deliveries, following his superb 115 in Australia's first innings.

Aged 20 years and 98 days, the NSW left-hander has broken a 79-year-old record previously held by George Headley. The West Indian scored twin centuries in the Guyana Test against England aged 20 years and 267 days in February 1930.

 AAP