Aircraft have landed on taxiways you know. Not that I'm advocating the Altn for Perth Rwy 21 is the parallel taxiway!!, but it could be used in a dire emergency as a last resort.
I'd worry more about un-forecast fog hitting Perth than an Aircraft becoming disabled on the runway..............
Most Cathay flights would arrive with about 3:00 hrs fuel ( 2:00 island reserve, 30 mins of un-used contingency and 30 mins fixed reserve ) and therefore could divert to another airport in WA should something unforseen occur. Qantas however? I don't know.