hello beanie
there are many flight search engines that allow you to pick multistop flights, dept london for ozz, jump on a flight to hong kong and then a flight back to the UK (3 flights only).
as an example only i tried opodo.co.uk and lastminute.com dept. 6/7 lhr - sydney, dept. 20/7 sydney - hong kong and dept. 27/7 hk - lhr. the cheapest flight price was with opodo flying with qantas at £1152 total price multi-leg incl. of taxes etc.....all legs were with 1 stop except hk - lhr non-stop.
at lastminute the cheapest came in at £1205 with virgin atlantic, with virgin the flight from sydney - hk was direct along with the hk - lhr flight. quotes were available with british airways on both sites with a starting price of approx £1220
there may be a few other sites/airlines to try and bring the price down, but rembember the price can vary depending on the time of year etc.... if you wish just reply with your dates, local uk airport, ozz airport and how many in your party with age at dept.(child, adult or senior citizen) i can have a quick look for you and reply within 24 hrs.
regards
jim