I believe this applies:
If your flight does not cross an IATA tariff area, but travels across a sub-tariff area then the first carrier that crosses the sub-tariff area is your most significant carrier
Since from
IATA tariff area 3
Asia, Guam and The Southwest Pacific
Sub-tariff areas in tariff area 3
Japan and Korea, Southeast Asia, The South Asian Sub-continent and Southwest Pacific
I believe that China and HK are in "Southeast Asia" and SYD is in Southwest Pacific, hence the HKG-SYD flight is the MSC flight and determines the base level of allowance for the whole journey (but the upgrade HGH-HKG gives you an extra 10kg for that sector only)..