Surgeons will avoid treating high-risk patients as a result of a rate-my-doctor website that hands great power to private insurers, warns the Australian Medical Association (AMA).

Private health insurers Bupa and HBF have agreed to join the Whitecoat doctor directory and customer review website founded by rival nib - expanding its reach to about six million privately insured Australians.

The health funds say Whitecoat will promote competition and improve transparency among high-charging doctors.

"There is simply too little transparency when it comes to the customer's experience of the healthcare system," nib managing director Mark Fitzgibbon said.

But the AMA opposes the online directory, claiming the private insurers will use it to dictate how and when people receive their healthcare, going against the very reason for having private health insurance - choice.

"The last thing the AMA wants to see is a United States-style managed care system in Australia where insurers, because of their huge market power, will have complete control over the care that patients receive," AMA President Dr Michael Gannon said.

Dubbed the 'TripAdvisor' for healthcare, Whitecoat was launched in 2013 and already carries publicly available information about more than 35,000 registered providers and hosts more than 250,000 customer reviews.

While the platform now includes allied healthcare providers such as dentists, physiotherapists and GPs, it is looking to expand to include a tool to rate medical specialists.

This contentious feature is a concern because its very difficult to produce data which takes into account everything regarding quality care, said Dr Gannon.

"If you use a blunt instrument just to report on re-admission rates or infection rates you might see doctors deferring on more complex care. We would never want to see that become the situation."

He says the best person to judge the suitability of a specialist and navigate you through the complexity of the private medical system is your GP.

"It's their job to hopefully know them and their background, to understand what they might specifically need and to help them make the choice of specialists," he said.