"My mother has on file many letters from Medibank Private informing her of other changes to premiums and the cover provided, but they seem to feel that it was not necessary to inform their clients of this very significant change," she said.

"Had we known that we were paying the gap for these investigations, we would have asked if all the tests were essential given the costs involved."

Ms Robins-Browne said after receiving the unexpected bill, a clerical manager at the diagnostic imaging company told her other Medibank customers had had similar experiences and that upon complaining, their gap fee was covered by the insurer. But so far, Ms Robins-Browne has had no such luck with Medibank and the diagnostic imaging company is now threatening to deploy debt collectors.

A spokeswoman for the Private Health Insurance Ombudsman said about 30 people had complained about the same issue and that all complaints had been resolved by Medibank without requiring further investigation by the ombudsman.

While the Private Health Insurance Act 2007 says insurers must notify members of changes that might be "detrimental" to the policy holder within a reasonable period of time before the changes take effect, the ombudsman's spokeswoman said Medibank Private had no obligation to inform members of this change because it was an alteration to its "agreements with health care providers".