This was announced by the company said on Wednesday.
Northam said in a statement the strike started with the overnight shift on Tuesday and that smelter operations were unaffected.
The company said it had legally challenged the stoppage in an urgent court application and was engaging with the National Union of Mineworkers (NUM), the majority union at the mine.
More than 7,000 NUM members went on an 11-week wage strike that ended in January last year at the mid-tier platinum producer.
The wider platinum industry is still recovering from a sometimes violent five-month strike last year by NUM's arch rival the Association of Mineworkers and Construction Union, which hit Anglo American Platinum, Impala Platinum and Lonmin, the world's top producers.