A proposed $6 co-payment for visits to a GP may also have to be rolled out in emergency departments to guard against clogging the hospital system, according to the former Coalition health adviser pushing the idea.

Terry Barnes, a policy adviser to Tony Abbott when he was the health minister, argued the GP co-payment proposal was a “fair and reasonable” way to ensure people value the health services they received. But his idea – being considered by the Abbott government-ordered commission of audit into government finances – has attracted criticism from the Australian Medical Association (AMA), the Labor opposition and the Greens.

The acting opposition leader, Penny Wong, said the “tax on taking a sick child to a doctor” was a “nasty surprise” for families in the lead-up to the new year. The government has refused to comment on its openness to adopt the co-payment scheme, saying it will not respond until formal commission recommendations are handed to the government next year.

Barnes said his proposal was for bulk-billed patients to pay a $6 co-payment each time they saw a GP, based on indexation of the short-lived $3.50 co-payment introduced by the Hawke government in the early 1990s.

Barnes said the taxpayer could not sustain growing Medicare costs. “What we are suggesting is that a small contribution by patients at the time they go and see a doctor puts more money in GPs’ pockets plus the Medicare rebate, keeps the whole system going,” he told ABC TV.

He responded to claims by critics that the fee could drive some people towards hospital emergency departments. “We think that $5 or $6 would not be enough to deter people from going to the doctor if they absolutely need to, and it has to be fair and reasonable as well, and that’s why we propose that no person on lower income or with children under 16 should pay a co-payment for more than 12 visits a year, but the average person goes to the doctor about six times a year – that’s $30, $40 – so that’s quite reasonable.

“In terms of emergency departments, I’ve seen the comments about that and I think the simple way to deal with it is actually to allow the states to charge a matching co-payment for people who do go to an ED,” he said.

Barnes said one of the motivations for the co-payment was to ensure people valued the health care they received.

“I think that when you have what to your wallet is a free good you don’t necessarily appreciate the full value, the full cost of what it takes to get that service. This is sending a price signal to people, there’s no question about that, but if it means that they know what they’re getting and that is access to a world-class healthcare system. Is that a bad thing?

“And the level of co-payment we’re suggesting is equivalent to a hamburger and fries, or a schooner of beer; I mean it’s not a great deal, and if we’re talking about say up to $50, $60, $70 a year max for people on low incomes, is that unreasonable?”

Wong said she believed most Australians accessed health care only when they needed it and the government should not create a disincentive for visiting a GP. She said Medicare was funded by the Medicare levy.

“There’s a principle here, which is accessibility of healthcare. We’ve got a system that’s been in place for decades which has worked well, which is regarded around the world as one of the best public healthcare systems that you can have, which ensures that Australians can access doctors, can access health services, not according to their ability to pay but according to need,” Wong told radio station 5AA,” she said.

“I think that’s a very important principle, and I think the government that is saying to Australians, we’re going to give tax breaks to large companies, we’re going to put in place a gold-plated paid parental leave scheme, but we’re going to hit you with a tax for going to the doctor, really has its priorities wrong.”

The health minister, Peter Dutton, acknowledged “widespread speculation” about health proposals, but said the government would not comment on “speculation around what the commission of audit may or may not recommend”.

“The Coalition government is committed to maintaining and improving our world-class health system,” he said. “The government has committed to funding in health and to making sure our health system is sustainable and accessible into the future. We stand by that commitment.”

Barnes said he had not spoken to Abbott or Dutton about the co-payment idea because it was a private, commissioned report. “One thing I do know having worked with Tony Abbott is that he would not endorse something which was not considered fair and reasonable,” Barnes said.

The AMA president, Steve Hambleton, said he was concerned about any proposal that prevented people going to a GP. He was particularly worried about disincentives for disadvantaged groups, including Indigenous people, pensioners and people with limited access.