Perhaps I could refer you to http://labourprotect.co.za/Working_Hours.html
On face value, it appears that you could be entitled to receive Sunday and public holiday adjustments. Note that the latter two are at 2 times the ordinary hours salary, although if you do work Sundays ordinarily, it will only be at 1.5 times the ordinary amount.
Furthermore, the time of day at which you work is not an indication of whether it is overtime. If you do work the night shift, however, you are entitled to a night shift allowance.