Medicare is the publicly funded Australian health system. It ensures a high level of health care for all Australians regardless of their level of income.
Medicare is not available to Temporary Residents, unless they are from Reciprocal Agreement countries, in which case the benefits are limited to "medically necessary" treatment.
It is for this reason that AHP exists. AHP and IMAN are here to service Temporary Residents and Overseas Visitors working in Australia.
For example, if a United States citizen were to have a heart attack in Australia not one cent of the cost of the surgery, hospitalisation, etc. would be borne by the Australian Government.
Their Sponsor, or their Employer, if that employer sponsored them, would be liable for all public hospital related expenses incurred in Australia. That is a very expensive risk to self-insure, so Sponsors require 457 VISA Employees to take out health insurance.