The excess deposits in Oz vary depending on who and where you hire from. Due to the nature of the roads in the outback, the car hire companies charge a larger amount for the deposit. The roads in the outback are classed as 'unsealed' so its more like driving on a large dirt road rather than concrete
It also depends on the size of the car your hiring...sounds to me like you are hiring a large sedan, convertible, a 4x4 or a people carrier?
Sounds to me like you have also got your hire with Europcar/National/Alamo or Hertz? If I remember correctly from my car hire days, they are the ones with the big deposits
Edit: Also a lot of the time they will not take the whole amount as a deposit. They will do a validation on the card to ensure that should they need to, they can take the money.
Be weary of Insurance4Carhire. They are a good company and yes we used to recommend them (unofficially) to people hiring in Oz, but you will still have to leave the security deposit.