A little late to this one, but a brief 5 mins on ITA matrix suggests you could do an open jaw SYD-MAD // ZAG-SYD via HKG and LON for less than AUD1700 plus AUD100 for the second stopover. Many combinations of CX, QF, BA and IB possible. I looked at random dates in Sep 13.

There are no direct flights between MAD and ZAG, but plenty of options shown on e.g. skyscanner.net, including EZY via LGW.

No need for an RTW.

hth