Well no, because there is no agreement between QF & SA.
So the only limitation is the bilateral civil aviation agreement between Australia and South Africa.
The
Australian Register of Available Capacity states that Australian carriers are entitled to operate 14 flights each way each week to Johannesburg, Capetown or Durban.
Services to other cities are unrestricted.
As far as I can tell QF are only using six slots per week, with QF63 SYD-JNB and QF64 JNB-SYD each operating every day except Wednesday.
So that would leave 8 slots each way each week open for use.