Dressed in the NUM's signature red and yellow T-shirts, members from various regions sang songs praising African National Congress Deputy President Cyril Ramaphosa.
Some waved their region's flags as they entered the stadium. NUM general secretary David Sipunzi was expected to report back to members following the union's central committee (CC) two-day meeting in Pretoria.
The NUM CC is the biggest decision-making body of the organisation outside congress.
Congress of South African Trade Unions (Cosatu) North West secretary Job Dliso said the decision by the NUM to choose the North West, particularly Rustenburg, to host the report back rally was appreciated.
"This shows how NUM is taking its members seriously and also implementing Cosatu’s 12th national congress resolutions of going back to the basics, touching base with its members, and giving them feedback on decisions taken both at the national level and Cosatu's recent central committee [meeting]," he said.
Closing the CC meeting in Pretoria, NUM president Thamsanqa Matosa said leadership battles in the union's branches were compromising service to members. "This situation triggers a decline in membership.
This possible decline will affect our majority in wage negotiations in 2018. We will be unable to direct the momentum of negotiations as usual. There needs to be urgent intervention in creating stability in our branches and recruiting more members to leverage our bargaining power," Matosa said.
The NUM lost members in Rustenburg during the August 2012 violent wildcat strikes which claimed 44 lives in the North West platinum belt.
The Association of Mineworkers and Construction Union (Amcu) gained most of these members and thus become the dominant union in platinum mines in Rustenburg.
Matosa, other NUM top leadership members, South African Communist Party North West secretary Madoda Sambatha, and Cosatu deputy secretary Solly Phetoe were among those who had already arrived at the stadium early on Saturday afternoon.