Nothing needs 'approval'. Because everyone has the same insurer (The government), if you present to a hospital you are triaged according to your risk and treated appropriately. No 'approval' and no 'rejection'. It just happens.
And if you have osteoarthritis of your knees and want surgery in the private system (which can generally happen on demand instead of - because it is an emergent/elective condition - some time between now and 12 months from now) then you can go ahead and organise that.
If you would like to read a media release by the College of Surgeons have a look at [1] which not only describes emergency and elective but talks about about how surgery is managed in the hospital.
whoops, missed the link. [1] http://www.surgeons.org/media/307115/sbm_2011-05-24_separati...