We visited South Africa in October last year and again in Feb/March this year.
(At the time), there was no need for Yellow Fever entering Sth Africa from Europe or Australia. OTOH, if entering from Zambia it WAS required for entry into Sth Africa and also coming back into Australia.
There was (and I understand still) is some confusion if transiting from Zambia to Australia via Sth Africa, as to whether you will require a Yellow Fiver certificate to merely transit JNB (as against a stopover). From memory, we were asked but not required to show the certificate; however we did need to show it in SYD (as we had been to Zambia) so we required the shot anyway.
So if only going to S Africa you should not need it (IME), but you should check with your doctor who will be able to give you the latest advice.
JV