Are you travelling straight through to Sydney, or are you having a stopover in Hong Kong?

If you are going straight through, then BA's baggage allowance should apply - and in practice, BA will through-check the bags to Sydney so you won't have to deal with QF people at HKG.

See this article for an overview of the current rules: http://www.businesstraveller.com/asi...ggage-policies; BA is the Most Significant Carrier for your journey, if you are going straight through.

I'm not sure how these rules work if you break your journey in Hong Kong and have a stopover there.

On the way back, you're on BA all the way so no issue arises.