Disclaimer: I'm not an expert, and this is what I've gathered to the best of my ability. I could be wrong in parts. I'm almost certainly missing some detail and have some terminology wrong.
If you end up in hospital for a procedure, you'll incur a number of costs. Insurance generally breaks these down into Hospital Expenses, and Medical Expenses. Hospital expenses are things like your bed on the ward, operating theatre fees, and administrative fees. Medical expenses are what your doctors charge - if it's an operation, that'll be your surgeon and the anaesthetist.
If you're a public patient at a public hospital, Medicare will pay for both of these. You shouldn't be out of pocket at all. But if you're a private patient at a public or private hospital, Medicare will only pay some. This is where private health insurance comes in. Private health insurance also means you