A couple thoughts with some digging (check out: https://www.awardhacker.com/)

Assumption: you are starting in the US. You have to get somewhere in Argentina / Chile for Antarctica cruise and you

1) US --> South America, 50k AS (can fly on either AS/LATAM or AS/AA or all AA)

2) South America --> Antarctica, RT cruise

3) South America --> Africa, 50k UA (from UR) getting you onto the GRU - JNB flight (plus inter-SA flights)

4) Africa --> Sydney, 78k SQ for JNB - SIN - SYD/AKL/BNE/MEL

5) Australia --> US, 55k AS if you can find QF J space, 60k AS if you can find CX J space via HKG, 90k UA from UR for *A through various destinations in Asia

6) Avios for any intra-SA flights that you can find on LATAM, Avios for any intra-Australia/New Zealand flights you can find on QF, Avios for any intra-Asia flights on CX or JL