maui...
My assertion is based on the fact that the aircraft will not be able to use the southern routes that Qantas use to fly close to great circle track (and to get out of the winds) as it is a twin engine aircraft, and the routes take you 4, sometimes 5 hours from an airport.
By flying further north, and having available airports (for ETOPS purposes), it will mean greatly increasing the distance flown.
Can the 777 make it non-stop SYD - JNB whilst complying with ETOPS requirements and not being severely payload restricted? That I don't know, but even if it CAN do it non stop, the additional miles coupled with stronger winds mean that it will take significantly longer.