As no one else has tackled this....
First of all it will depend on what you are booking. Ticket will be charged in local currency of departure port, so anything booked out of HKG will be in $HKD and charged from HKG, so you will be hit for the transaction fee for sure - and the same is likely to apply if booking anything other than either a return or one way originating from Australia.
Originating from Australia, you will be charged in $AUD. That is as far as I can advise. I can't immediately check exactly where the merchant location registers. I used Amex, so charged in $AUD had no currency conversion fee, but can't speak for other banks that do the sneaky charge based on merchant location.