(Adds detail, background)
JOHANNESBURG, Sept 11 (Reuters) - South Africa's Chamber of Mines said on Wednesday that wage settlements in the gold sector would amount to 1.5 billion rand ($150 million) in extra costs for companies over the next 12 months.
Companies and unions agreed to wage hikes of up to eight percent last week, ending a three-day strike.
Hardline union AMCU, which represents almost 20 percent of the gold workforce, has not signed up and said its members have given it a mandate to strike.
But Elize Strydom, the chamber's chief negotiator, said that since most of the workforce had signed legally binding agreements, companies can apply the wage settlement unilaterally to all workers.
Any strike would be considered illegal and workers taking part could be fired. ($1 = 9.9784 South African rand) (Reporting by Ed Stoddard; Editing by Jon Herskovitz)