Also consider CX - via HKG. There is an excellent connection (only 1.5 hours) by taking CX 104 and connecting to CX749. There are also other options available via HKG on some days.

Not sure as to the cost.

You may want to consider a OneWorld Circle Indian Ocean Fare - would allow you to get to JNB via HKG on way and direct back on QF. The OneWorld fare tends to be cheaper than last minute point to point fares, and in J has no advance purchase requirement. Ask your travel agent or contact QF directly. For more help on the Oneworld fare hop on over to the OneWorld forum.

Hope things work out for you.