In 2004 around 540,000 South Africans contracted HIV, but the number dropped by a third to 370,000 in 2012, UNAIDS said.
"The general message is that it's good news," said the body's country coordinator Catherine Sozi.
Graphs showed plummeting numbers of people dying of the condition yearly from 330,000 to 240,000 in the same period.
At 6.4 million seropositive cases South Africa still has the largest number of people living with AIDS, but the figures showed success in public treatment programmes, according to Sozi.
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Around 2.4 million people are receiving free AIDS drugs today.
Transmission from mothers to their babies fell the sharpest and is now approaching zero, according to UNAIDS.
The latest figures brings the country closer to its target of halving new infections.