Actually, in your link above, iPhone 4 prices start from $859. $719 is the price of the 3GS.
I don't know how it is in Australia, but the contract price discrepancy between Android phones and iPhones is well known in England. For example, O2 sells the Galaxy S2 for free with a 24 month £42 plan, while the 16GB iPhone 4 is free on a 24 month £67 plan.
The Galaxy S2 plan gives you a bit more data, and you do have more flexibility with Galaxy S2 plans - you can choose to pay more for the phone and less for the plan for example. (Of course, you also have a big choice of cheaper Android phones too).
Here, plans are pretty comparable.
Galaxy S2, $5/month on $59 plan: http://www.optus.com.au/store/phone/galaxysii?sid=MobAFeat1:...
iPhone 4 16GB, $7/month on $59 plan: http://www.optus.com.au/store/iphone/plans
The Galaxy S2 plan gives you a bit more data, and you do have more flexibility with Galaxy S2 plans - you can choose to pay more for the phone and less for the plan for example. (Of course, you also have a big choice of cheaper Android phones too).