The health department can't predict whether the federal government's freeze on Medicare rebates will leave patients facing higher out-of-pocket costs to see a doctor.
Tuesday's budget extended by two years to 2020 a four-year indexation freeze on the Medicare rebate the federal government pays for services like GP visits, saving almost $1 billion.
Doctors say they can't keep absorbing the costs, meaning less will bulk-bill and fees will rise.
Department deputy secretary Andrew Stuart on Friday said bulk-billing rates for GP services were at an all-time high of 84.3 per cent, having continued to rise about one per cent each year even after the freeze began in 2013.
But asked during a Senate estimates hearing in Canberra whether the government's plan to extend the freeze would impact future bulk-billing rates and out-of-pocket costs for patients, Mr Stuart said it was "very difficult for the department to predict any precise outcomes".
He said GPs could respond to the freeze in a range of ways, including accepting a cut to income or working extra hours.
For patients who aren't bulk-billed, out-of-pocket costs have continued to increase by an average of five or six per cent every year as they have for most of the decade, he said.
"We've seen no variation on what's been happening over the last decade at this point," department boss Martin Bowles said.
Doctors are warning of a fierce campaign against the freeze in the lead-up to a July 2 election, bigger than the one that forced the federal government to declare its short-lived GP co-payment proposal "dead, buried and cremated" within months.
Rural Doctors Association president Ewen McPhee says the freeze has already forced doctors in some towns to start charging patients while others have told him they will shut up shop after Tuesday's announcement because they can't afford to function any more.
The Australian Medical Association has said the poorest, sickest and most vulnerable Australians will be hardest hit.
But Rural Health Minister Fiona Nash on Friday insisted difficult decisions had to be made.
"We are in a process of budget repair," she said.
She said the government remained committed to a possible review of the pause as further improvements and inefficiencies are identified.
Health Minister Sussan Ley has previously refused to rule out the possibility of costs being passed on to patients, insisting her department's modelling shows changes will be minimal.
