The Australian Medical Association (AMA) has expressed its formal opposition to the Federal Government's new co-payment model, labelling it a "wrecking ball".

The Government is proposing to introduce a $5 reduction in Medicare rebates to doctors for GP consultations.

AMA president Brian Owler said the $5 reduction was unacceptable and did not recognise the contribution GPs make to society.

"A lot of GPs felt very insulted by the fact that even though they're experienced GPs and can be efficient ... dealing sometimes with complex problems, the rebate they were going to be given for those was going to be slashed from $37 down to $11," he said.

Doctors also reacted strongly to Government plans to introduce the new co-payment system by the middle of next month.

"That this should be instituted and ready to go by January 19 is, I think, absurd," Associate Professor Owler said.

"Particularly when there has been absolutely no consultation on this issue."

The Abbott Government earlier this month toned down its plan to make Australians pay $7 every time they visited the doctor.

The Prime Minister announced last week exemptions for veterans, pensioners, children and people on health cards and changed the amount to $5.

But, in a major twist, the $5 would be cut from the rebate paid to doctors, allowing the Government to argue it was up to GPs whether to charge patients for a visit.

Crossbench senator Nick Xenophon claimed the new plan was tantamount to declaring war on doctors.