For some perspective, the QF 9 when it flies Melbourne to Perth, which is about a 3 and a half hour flight, will burn roughly an extra 100kg of fuel per 1000kg of extra weight (fuel/bags/passengers/freight). On the 18 hour leg from Perth to London it will burn an extra 600kg of fuel per 1000kg of weight. With a fuel load of about 100,000kg you can see how carrying the extra fuel to fly so far means you burn an awful lot of it just to carry the weight of the fuel. So clearly a stop half way would, from purely a fuel burn perspective, be a more efficient journey.