What a few people here aren't able to understand (and what I think JamieMaree is trying to explain) is that this diversion (in this case back to Perth) is nothing to do with ETOPS/EDTO. ETOPS just allows the flight to operate further from an alternate airport than usual. The diversion is usually due to a statement in a checklist which says something to the effect of "Land At The Nearest Suitable Airport" and would take place regardless of the sector being ETOPS or non-ETOPS.
On a twin engine aircraft this statement is usually included in the checklists involving in-flight engine shutdown. If the capt didn't shut down the engine but left it running at idle (maybe because he wanted the GEN and HYD systems available) there's an argument that you don't need to Land At The Nearest Suitable Airport. I hope you guys understand now.
I personally wouldn't feel very comfortable flying past YPLM to continue hundreds of NM to YPPH with an engine running but incapable of delivering power above idle but that's a different argument.