If the airport was in ground stop and the ramp closed, it is possible that they couldn't get fueling equipment in place to fully fuel the airplane in SYD
as they were rushing ahead of the curfew.
I've seen it happen when on rare occasion an airplane will dispatch without enough fuel for its destination and file an amended flight plan to make a stopover for fuel. This usually happens when something is preventing aircraft from being fully fueled. An example was at MIA
when they had their fuel station fire and while fueling was possible it would take hours to fill a widebody with fuel. However making it all the way to CMB isn't typical if the problem was fueling equipment and availability at SYD
. Usually it would be a quick stopover at an airport close by.
If you have never designed an airplane part before, let the real designers do the work!