A group of non-GP specialists has been accused of charging private patients up to $5000 in booking and administrative fees while signing up to no-gap fee schemes with health insurers.

The claims, published in The Age newspaper last week, come from an unnamed whistleblower who said she reported the activities to her superior working at the clinic but was told it had been common practice for more than a decade.

Her letter sent to Department of Health and Aged Care investigators refers to many of the doctors by name and claims the practice she worked had charged most of its patients between $500 and $5000 out-of-pocket costs.

Health insurers say they are also investigating, declaring that the allegations had been made against nearly 50 doctors, mainly anaesthetists and surgeons.