"Eight of the nine mineworkers who were missing at Doornkop Mine are found dead," the union said in a statement. A search was ongoing for the ninth man still missing.
The union said the bodies were discovered last night by rescue teams.
Initially 17 workers were trapped underground after a fire broke out about 1.7 kilometres beneath the surface at mining firm Harmony Gold's Doornkop operations late Tuesday, in an accident that may have been triggered by a seismic event.
The union said the damage to electric cables had triggered a fire underground after a rock fall.
Despite efforts to reduce fatalities, accidents are still common in South Africa's ultra-deep gold mines.
One hundred and twenty eight people were killed in 2010, according to the latest figures published by local NGO Miningsafety.Co.Za.