The answer is main rounder argument: Jetstreams.
Flight time of Johannesburg to sydney is 11 hours 25 minutes. This is okey.
http://tr.flightaware.com/live/flight/QFA64/history/20170502/1710Z/FAOR/YSSY
But the opposite path:
Sydney to Johannesburg is 14 hours and 10 minutes.
http://tr.flightaware.com/live/flight/QFA63/history/20170502/0050Z/YSSY/FAOR
This is real flight time. The opposite route is manipulated by jetstreams
This is to be expected. But we are comparing North Hemisphere to South Hemisphere, not South Hemisphere to South Hemisphere while adjusting for Jet Streams.
As an example: The distance from New York City - London is ~3500 miles. The distance from Buenos Aires - Sydney is ~ 7300 miles. But if you look at the flat Earth Map the distance from Buenos Aires - Sydney should be 9.5x the distance, not 2x the distance as NYC - London if the map is accurate. If you take the most "efficient" flight path on the Flat Earth, it would only be 6x the distance but then you would literally fly over North America which NEVER happens, I've flown to and from Sydney and various South America locations and we never have flown over North America.
Now on to your likely rebuttal. The Flat Earth map is just a representation of what the flat earth society thinks it should look like. No actual accurate flat earth map exists because....... Why? Why has no professional geographer, cartographer or pilot produced a verifiable accurate map? One MUST exist if flight paths, shipping paths, international borders, date lines and time zones are to be adhered to and yet all we "Round Earth Believers" can do is trust that they aren't being shown to us? I'm not sure how you can convince anyone, even yourselves that what you are suggesting is even remotely true if you haven't seen what the Earth actually looks like.