It is understood that the group are from rival gangs who were working underground to mine gold in abandoned shafts.
Several police units and other law enforcement officers have been deployed to the Springs area, east of Johannesburg to monitor the situation.
Police say the victims were shot dead using different calibres of guns.
The authorities fear that the body count of the illegal miners known as “amazama-zama”, meaning those who hustle, will increase as they continue to work on scene.
The violence underground appears to be part of a growing problem, which the government and the mining industry are battling to cope with, says the BBC’s Nomsa Maseko in Johannesburg.
Johannesburg is dotted with disused mine shafts, which attract men from around the region, including Lesotho, Mozambique and Zimbabwe, with the promise of discovering remaining gold deposits.
Abandoned shafts become war zones when rival gangs clash – they sometimes throw improvised bombs at one another and open fire with machine guns, our correspondent says.
According to South Africa’s Department of Mineral Resources, a 2008 study of the gold sector found that an estimated $509m (£330m) in revenue was lost a year as a result of illegal mining.
South Africa has some of the world’s deepest gold mines and safety is a a big problem.