I'm not sure who does it, but who ever works out if fuel should be ferried (someone in an office>) ... or amount of fuel carried (Assume Captains role) must have some pretty sophisticated formulas to try and make sure these 'beans' are being efficiently used, in the safest manner, with ever changing weather conditions. I'm guessing / hoping it's not an excel sheet.
In this case above, I'm guessing he was ferrying fuel for the return leg? Or would Sydney have been the alternate no matter what, and the fuel carried calculated based on that. (That means, no ferrying, right? As you would need to carry the normal load, plus the diversion, plus the return leg?). Wow... it might be pretty complex.