President Jacob Zuma must apologise for fathering an illegitimate child and strive to set a better example, says Democratic Alliance leader Helen Zille.

Zille said Zuma's behaviour was at odds with the African National Congress's policies on HIV prevention.

The ruling party was wrong to say his reported extra-marital relationship with the daughter of Orlando Pirates boss Irvin Khoza was purely a private matter, she said.

"For this reason, the president ought to apologise and act to better embody the values he advocates for other South Africans."

The ANC on Monday rushed to Zuma's defence amid outrage over the affair, which Zille and other critics said would damage the fight against HIV/Aids in the country, which has the world's highest number of victims of the disease.

The party said it did not see a correlation between Zuma's personal relationships and the ANC's policies on HIV and Aids.

"There is nothing wrong that the president has done," said party spokesman Brian Mthembu.

"As the ANC, we have always made a distinction between people's personal affairs and their public responsibilities."

Zille countered that elected public officials had to embody the principles and values for which they stand.

"They embody a series of principles and should be able to explain their actions in terms of those principles. Jacob Zuma cannot do this," she said.

"If Jacob Zuma says unprotected sex with multiple partners is bad in public, he is expected to uphold these values in private, otherwise there is little or no reason to take seriously anything he or his government says."

After he was acquitted of rape in 2006, Zuma apologised for having unprotected sex with an HIV-positive daughter of a family friend. The judge found that he had consensual sex with the complainant.

"I wish to state categorically and place on record that I erred in having unprotected sex. I should have known better and I should have acted with greater caution and responsibility," he said.

Zille said that although she believed he should apologise again, this might ring hollow.

"The only question is whether, if he apologises this time around, anyone will actually believe him."