I did it in January this year. It was the tail end of an ex MRU AONE and a BA redemption flight back to LHR.

I had issues with the redemption ticket as more taxes needed to be extracted by BA which meant going through immigrations at JNB as this could only be done landside.

If you aren't able to secure a BA boarding pass in SYD you might have to wait in the none to nice transit area at JNB until the BA people show up.

My luggage was through checked without a problem.

My TR is here