Note that your osteopath may charge a ‘gap fee’ (this is the difference between the rebate the health insurer offers you and the total amount of the service). If you are not happy with the level of the rebate, then speak to your health insurer, as the osteopath has no control over the rebate the insurer gives you.
The Australian Government Private Health Insurance Ombudsman provides general information and a listing of health funds at www.privatehealth.gov.au. It also has a health insurance policy comparison tool.