Depends on where you depart from in South America. *A is pretty weak there, so you will end up doing a long route whatever happens.

From SCL for example there are only flights to YYZ on AC
From LIM there are only flights to YYZ on AC
From EZE there are flights to IAD on UA
From GIG & GRU there are flights to ORD and IAD on UA

Availability on the AC flights in particular can be tough to find.

There is also a flight from GRU to JNB on SA, but this won't save any time as you then have to route via Asia to then get to Australia.

However you look at it, you've got quite a journey which in turn gives you a chance for a decent stopover opportunity or two. Have a look at the *A timetable as FOM suggests and see what suits.