With petrol surging and public transport easily hitting A$12 a return trip, plus the associated time, it's easy to see why.
Considering Zone2 is aspirational for many, not to mention those who'd rather live in the home counties (i.e. 'suburbs' I suppose) and get a certainly much more expensive train in (and then the tube) I don't find $12 questionable at all. But markets do differ! Interested to hear if that really is outrageous sounding in Australia.
Also, it takes 1.5 hours on the train each way. This is only 42 miles platform to platform. Also, no public transit syncs with the train on either end.