Hi Allan C,
The reason why Qantas would only be coming up with that round-about routing is because there are only so many partner airlines that you can use to get a reward seat, and clearly on the dates you need it there isn’t one available to get you the whole way there anyway other than via Sydney.
Award seats are not made available on all flights or dates, and even multi-cities are no guarantee of getting the entire itinerary. Qantas fly perth to Singapore, but then from Singapore to Cape Town SQ are star alliance so no seats would show. The system would basically search a multi-city combo involving some ludicrous route trying to find seats on Qatar, Emirates or Cathay to get you there.
My advice would be to search for just part of the itinerary using points and to look at maybe paying cash for certain components. Try searching SIN - CPT - PER and see if anything comes up and you could pay a cash fare to Singapore to join it all up. Similarly you could search PER - CPT or SIN - CPT with points, as cash fares originating in South Africa are comparatively very cheap to cash fares originating here.
Best of luck!