"457 visa holders are usually required to purchase their own since everyone else is covered under the national health care system."
Not sure if you are aware of this, but if you are not entitled to Medicare, you are exempt from paying Medicare levy (and surcharge), which the rest of the residents do have to pay. Though, you have to get it back with tax refund at the end of the financial year (it is paid with PAYG taxes) after you apply for a letter from Medicare that you weren't eligible.
Private hospital insurance should cost you about the same per year, so it's a fair arrangement, I think.