One day after the federal government dumped its cut to the Medicare rebate for short GP consultations of $20, the Victorian government stepped up its attack on the broader $5 co-payment for all GP visits, due to start on July 1.

Deputy Premier James Merlino said modelling showed the $5 co-payment would deter many Victorians from visiting their GPs because they could not afford it, making them sick enough to need hospital emergency care.

"As it is, we know that around 7 per cent of sick people delay going to a GP due to cost… Our modelling shows that 10,000 people each and every year will be forced to go to an emergency department because of Tony Abbott's GP tax," he said.

Under its controversial co-payment plan, the federal government will cut the Medicare rebate by $5 for all GP consultations. It is also freezing indexation of Medicare rebates designed to keep up with the rising costs of doing business.

While GPs could absorb the extra costs, and take a pay cut of $5 per visit, the Australian Medical Association says most will pass the loss on as a new fee to patients because they would not be able to afford the hit.

While children under 16 and those with a concession card are exempt, doctors say the plan, together with the indexation freeze, will prompt many bulk billing doctors to start charging private fees.

They say this would hit those on low incomes without a concession the hardest.

A spokesman for the Australasian College for Emergency Medicine, Dr Simon Judkins, said the Victorian government's modelling confirmed emergency doctors' fears that the plan would drive people to crowded hospitals.

"Most emergency departments are already overrun. We know that presentations on average increase 5-8 per cent every year, so why put a policy in place which will potentially see that number higher?" he said.

Dr Judkins said emergency departments already saw patients who could not get into see a GP, especially after hours. Hospitals were also seeing large numbers of elderly patients who cannot access GPs in nursing homes, he said, and the plan overlooked the value of GPs keeping people healthy and out of hospitals, where treatment is more expensive.

If the plan goes ahead Dr Judkins said thousands of extra patients would likely blow out waiting times for people seeking emergency care, making it harder for staff to swiftly treat the sickest patients.

"In an environment where decisions need to be made which can have serious consequences, we need to decrease the stress and distractions, not add to them," he said.

Mr Merlino said the Victorian government would fight the co-payment to protect hospitals from the additional burden it would put on them.

"Our health system is in distress, the last thing it needs is Tony Abbott getting away with a GP tax slug… This will be a fight we pursue with Tony Abbott and his government on behalf of all Victorians," he said.

In 2013-14, Victorian emergency departments, which are mostly funded by the state government, saw about 1.5 million patients.

A spokesman for federal health minister, Sussan Ley, said although a modest co-payment was necessary to ensure the sustainability of Medicare, she was happy to receive a submission from the Victorian government about its figures.