I have always wondered if IATA keep some statistics of which airlines are most prone to bump passengers?
Also, most bumped passengers get rerouted. In this case, rerouting would be difficult, and it would probably be quicker to wait until the next flight. A reroute via Sydney would have required a flight on the following day (assuming there was one since Qantas don't fly Sydney-Joburg daily). Other reroutes could include Hong Kong and Singapore, but given the time SA281 leaves Perth (23:45) that would also mean waiting until the next day!
SAA is represented by a local Australian ground handling company. However, the instructions regarding who to bump, how to do it, and what arrangements to make come from Operations in Johannesburg.