Since the world apparently isn't flat the actual difference in distance between SYD-LHR and AKL-LHR is actually very significant. You're looking about required range increases well beyond what is feasible with current aircraft designs.

Once you hit that ~20hr mark there is also the very big question of economics. The fuel load required to get that far means an incredibly heavy plane for the first ~6 hours of the flight, and that heavy weight means more fuel burn. You're in a situation where you need to carry extra fuel just to carry the fuel required to fly that far.