I disagree - Cathay's website contains details on this area: http://www.cathaypacific.com/cx/en_H...ier-rules.html

From which, even if both sectors on a "single journey" are CX, the one that determines the baggage allowance for the whole trip is the Most Significant Carrier sector, and in your case that is HKG-SYD.

So on my reading of the rules you would only be allowed 30kg for the HKG-SYD sector.

EDIT - but wait - you're a CX DM according to your profile - that means you get 50kg even in Economy!