Google is your friend. An "open jaw" booking is where you fly to one city and home from a second. In your case, to ADL, home from SYD.

I'm saying that before you automatically book LHR-ADL-SYD-LHR, try just booking the long legs as an open jaw, then price the domestic ADL-SYD one-way flight separately (it can cost as little as AUD$55 or so on Tiger). The net total could be cheaper.