Just ticketing SYD-NRT-KHH.....TPE-HKG-SIN-SYD, and asked the same question of QF. In the rules it states if there is a change after you begin the jouney, then a US$125 fee occurs plus the local airline surcharge for any changes requiring re-issue of tickets, which includes E-tickets.
I was enquiring about dropping the SIN leg of my journey incase I was held over in Hong Kong. Answer I received was Hong Kong office does not charge a feeshock: :shock: :shock: ), but the US$125 still applies, less or plus taxes etc for your change. Interstingly enough, QF in Oz charges a fee.....:evil: