Brigadier Thulani Ngubane said that the 46-year-old man was killed at Billy's Tavern, near Anglo Platinum's Khomanani mine shaft two, just before 1pm.
"The deceased, who is alleged to be the regional organiser of Amcu in the Rustenburg platinum belt, was watching soccer and having a good time with his friends in the local tavern."
Four men in blue overalls came into the tavern and one of them shot the man four times in the back, at close rang, with a 9mm pistol, said Ngubane.
"The [Amcu official] died on the scene, all other people who were with him in the tavern ran for cover," Ngubane said.
The suspects fled the scene on foot and no arrests had been made by 9pm on Saturday.
ENews Channel Africa reported earlier that the man, who cannot be named until his family have been told of his death, had been expected to testify before the Marikana Commission of Inquiry.
The commission is investigating the deaths of 44 people killed in violence related to a strike at Lonmin's Marikana mine in August last year. – Sapa