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.