Having said all that, I also wonder as how you (@ cheltzel) obtained that (very optimistic) 5.100 nmi distance between IST and SYD. Indeed (international) 5.100 nmi gives you the (km) equivalent of 9.445 km. However the airport to airport distances (not the applicable flight paths) between IST & SYD is very near to 15.000 km and between IST & MEL is 14.700 km. SQ's a350-900ULR can barely make it (with minimal reserves) but that's a different story. Once the 350-900ULR crosses the Pacific (still taking a polar route, I guess) and starts flying over the States, there are plenty of airports it can divert (and accommodate this type aircraft) in case of an emergency. On the way back, this is less so... But in case of IST - SYD/MEL route, the chances may be less favorable.