Hiya
We're not in Oz yet but are also looking into private health insurance at the moment. If you haven't already seen it, there's some useful info. by the Private Health Insurance Admin Council at
http://www.phiac.gov.au/
From what I understand, Medicare will cover you for "necessary" treatment under the reciprocal agreement with the UK, but I'm not entirely sure what this does actually cover. If I find out shortly, I'll let you know!
Just another thought, in SA at least you might want to consider ambulance cover. I think it's $107 p.a. per family and saves paying $600 if you need to use one and haven't got cover. Some health funds offer this cover.
Hope this helps
Bex