Originally Posted by mafaky

When the airplane manufacturer states the "range is (xyz) nm." this is an ideal condition. It doesn't take into account:
a) the adverse effects of head winds that will cause the plane will burn more fuel. (e.g. from SYD/MEL to IST there will be considerable head winds...)
b) It assumes the flight will be carried wholly at an altitude where the plane burns minimum fuel.
c) It may not take into account the MTOW specification (partly valid), but perhaps a more conservative figure,
d) It cannot take into account the reserve fuel needed )or the extra distance to be travelled) in the unlucky case that the flight needs to divert to a different airport instead of the destined one.

So taking these conditions into account, the airline needs to calculate an applicable range to be on the safer side. I am not familiar with the rules of thumb, but I presume that it may be 75%-80% of the manufacturer's claimed specifications (even when taking off at MTOW).

Futhermore, in case of IST-SYD/MEL (and return) flights one airport is on the northern hemisphere and the others in the southern. So, there's no chance of making short cut routes (like the "over the pole" option when both airports are on the same (mostly) northern hemisphere. Also that 5.1000 nm that you mention may not be the actual applicable air flight path and distance. Commercial flights still have to make use of certain pre-determined air corridors; they cannot fly fly at any shortest and direct direction to reach their destination...