An estimated 12 million people -- nearly a fifth of the population -- may have contracted coronavirus in South Africa, the health minister says, as the country records significant declines in new infections.

South Africa has so far registered 650,749 cases or 47.8 percent of the total numbers recorded in Africa. At least 15,499 of those infected have died.

But the actual numbers of people who got infected could be much higher, possibly 18-fold more, based on estimates extrapolated from sample antibodies studies. 

Revised models "currently predict that there are probably about 12 million" South Africans with detected or undetected coronavirus, Health Minister Zweli Mkhize said in a statement released overnight.

"This translates to about 20% of the population." 

South Africa is conducting a national study aimed at providing accurate figures about the prevalence of coronavirus antibodies -- a sign of infection -- among its 58 million people.

Mkhize said the drop in numbers of daily detected infections "raises the question of the level of immunity that may already be existing in society". 

Leading vaccinologist Shabir Madhi, a professor at the University  of the Witwatersrand, suggested infections rates could be as high as 40 percent in densely-populated parts of the country. 

"What we have seen in South Africa -- at least based on early data ... is that in densely-populated areas of the country... between 35 to 40 percent of people have been infected with this virus," Madhi, who is also in charge of the South African leg of the Oxford COVID-19 vaccine trial, told an online meeting on Sunday.

Case numbers in South Africa have fallen from 10,000-15,000 a day in July to just under 2,000 in recent days.

On Monday the number of daily new infections plunged to 956, the second time in under a week that the numbers dropped below a 1,000.