I wouldn't exactly say that Australia to South Africa (JNB) is easy on Qantas/Oneworld.

Three flights a week only on Qantas without going through Asia. 17-20 hours each depending if you go via Harare or direct.

SQ does daily (?) flights and they only take 24 hr SYD-SIN-JNB. More flexibility there. The key is SQ is not in Star or Oneworld yet.

Thai does 3 weeklies from BKK to JNB but the connect from SYD is short (half hour). If you only have carry on maybe? Time would not be much different than longer QF flights (ie. 21 hour total travel time).

I wouldn't call any but the SQ service to be adequate.