As far as I know, not every medical insurance for short visitors. Some local (state) health insurances do not provide short term insurance for visitors.
My grandma came to visit me last year, I bought health insurance for her from medibank. They are good.
You don't need to worry medical levy surcharge, only when your family taxable income is over $100,000 per financial year, you will be required to pay extra 1.5% medical levy surcharge on top of 1% medical levy. And, the 1% medical levy is necessary to pay even you have your private health insurance.