Labor says a $5 fee for bulk-billed GP visits would put extra pressure on the public health system by clogging up hospital emergency departments, labelling the idea a "tax on taking your sick child to the doctor".
The federal government has refused to "speculate" about a proposal to make patients pay the fee after a national Commission of Audit received a proposal for a co-payment scheme for GP consultations.
But doctor groups have criticised such a move, claiming it would destroy Medicare and limit access to GPs for some community groups.
Speaking on Sunday, acting opposition leader Penny Wong said a "real problem" with the approach was that it risked "clogging up our emergency departments in our public hospital system".
"What we don't need is a new tax on taking your family to the doctor and what we don't need is more pressure on our public hospital system," senator Wong said in Adelaide.
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She said the idea represented "another broken promise from this Prime Minister".
"He (Tony Abbott) said no new taxes and now today we've got a tax on sick children going to the doctor."
News Corp Australia reports co-payments would see patients pay a fee for bulk-billed GP consultations, with pensioners and concession card holders exempt from the fee.
Families would also be granted up to 12 bulk-billed visits annually.
Federal Health Minister Peter Dutton said he would not comment on "speculation around what the Commission of Audit may or may not recommend".
"The commission's work is still being compiled and will be provided to the government in 2014," he said in a statement.
"The government will be able to consider any recommendations and respond after that time."
Mr Dutton said the government had committed to making sure the health system was sustainable and accessible in the future.
Acting Greens leader Senator Richard Di Natale said on Sunday it was a step towards a two-tiered US-style health system and away from universal healthcare.
Dr Con Costa, of the Doctors' Reform Society, said a GP fee would make emergency departments busier.
"Introducing a $5 co-payment is false cost-savings," he said in a statement.
Australian Medical Association (AMA) president Dr Steve Hambleton said the majority of visits to the GP were very "reasonable and helpful" with only a minority causing doctors to ask why they came.
A co-payment scheme was introduced under the Hawke government in the 1990s but was scrapped.
