BA.com covers this in detail http://www.britishairways.com/travel...k/public/en_gb

"Baggage allowances on codeshare, oneworld and alliance partners may differ. Charges may apply" (as you suspected)

If you fly BA metal to SIN and then QF SIN-MEL then BA will allow their baggage allowance and QF will accept it for the interline at SIN; however on the return trip, check-in at MEL, you will be faced with the QF limit and charges for any excess. Unfortunately Oneworld doesn't offer any baggage benefits in this regard (status or otherwise).