I don't think you can fly TG BKK-JNB on a STAR Alliance RTW - its an SA-operated flight isn't it? Non-Alliance operated codeshares are not allowed. Since Varig pulled out of BKK-JNB-GRU last year the only way to Africa from Australia is via FRA - I did it once, a very long detour.
If you have any spare QANTAS FF points you could take a one way to JNB and buy your ticket there. Until the end of September there's a 30% discount on points needed for Australia - Africa so SYD-JNB is 42000 and PER-JNB only 28000 points oneway EC.