To add to what
wandering_fred posted (G'Day Fred)
The actual wording of the relevant part of the fare rules is:
When travel originates in a country for which a specific local currency fares is published and the ticket is sold in another country, the fare will be that published for the country of origin converted to the currency of the country of sale at the bank selling rate. The resultant fare must not be lower than from the country of sale.
Exception: Not applicable when BOTH travel originates and sales are made within Europe.
So the ex-Australia fare is converted from AUD to ZAR.
If the resultant fare is less than the ex-South Africa fare, then the ex-South Africa fare is charged
Otherwise the ex-Australia fare is charged.
Since ex-South Africa fares are significantly lower than ex-Australia fares, you will end up paying the Australian fare. So there would be no saving by purchasing the fare from South Africa.
As Fred suggested, purchasing an ex-South Africa fare is the way to go - especially if you are going to be there anyway - but YMMV depending on your planned itinerary (which may not include Africa at all, and which may not have enough 'spare' segments for the flights out of and back in to Africa).
Edited to add:
So to answer your specific question - the ex-Australia DONE4 base fare is AUD10,499, which converts to about ZAR120,000