I checked for a non-stop from Buenos Aires to Sydney about a week ago and found 1 that was 19 hours.  Currently only a 1 stop.  Most flights stop in  Dubai.  Why?  You might ask (It is in the straight line path on the FE map).  Here is the only currently available 1 stop flight (through Dallas):
Depart:    Buenos Aires - Ministro Pistarini (EZE) to Sydney - Kingsford Smith (SYD)
   
   American Airlines
Flight 996     Departs: Thu Feb 11 9:20pm    Buenos Aires - Ministro Pistarini (EZE)
Arrives: Fri Feb 12 5:24am    Dallas - TX, Dallas/Fort Worth International (DFW)
   Flight Duration: 11h 04m
Connection Time:  14h 46m
      
   
   American Airlines
Flight 7375     Departs: Fri Feb 12 8:10pm    Dallas - TX, Dallas/Fort Worth International (DFW)
Arrives: Sun Feb 14 6:05am    Sydney - Kingsford Smith (SYD)
   Flight Duration: 16h 55m
      
Operated by: QANTAS AIRWAYS LIMITED