Because the flight stops in Brisbane, have them try looking for DFW-BNE and then BNE-SYD. It appears there may be more availability DFW-SYD than there is DFW-BNE so depending on what day you are looking for and how the AA agent is looking at the flights, have them look at both DFW-SYD and DFW-BNE and BNE-SYD.
What day are you looking at?