If the crew oxy system was pressurised prior to the shutoff valve on the bottle (not the overhead switch) being closed on the A330 you can do the press test several times on the mask assy before the low pressure side of the system drops low enough to give you the ECAM pop up and indication on the SD door page.
Its quite feasable that the crew did thier test in MNL all looked good but when the crew in SYD did their test the pressure dropped to the trigger point and the rest is history.
One good thing came out of this.
We know EBA oxy system does NOT leak