We have 2 kids in NSW who have been in state & private schools. In Public state schools in NSW on a 457 you will have to pay the $4,500 as indicated by Northshorepom, plus other costs of around $400 per year each for things like building funds, library costs etc. Best option is if you can get the company to pay for it. A private school may be comparable in fees but there are other hidden costs such as text books & the uniform can be expensive. Another expense can be the bus pass (applies to both schools), this will depend on a range of things though.
To be honest i dn't know how the Aussies afford so many kids & school fees!