The Australian Medical Association says under the new "10 minute rule" the $20 difference in government subsidies is likely to be passed on – meaning some patients who have previously seen their GP for free will now pay. The move comes into effect on January 19 but will require the Parliament's endorsement when it first meets in February. It is guaranteed to pass the lower house because the government commands a majority in the House of Representatives. But there are signs it could be struck down in the Senate, with senators Jacqui Lambie, Ricky Muir and Nick Xenophon voicing strong opposition to the changes. Independent senator Jacqui Lambie revealed on Tuesday that she would vote to strike down the backdoor $20 GP fee. "Abbott's decision will mean that less people visit their GPs and it will mean that more serious illnesses will not be detected earlier," she said.

Senator Lambie called on Tasmanian Liberals to declare their positions on the rebate cuts and said voters in Queensland should use the state election to send a message to the federal Coalition on the issue. Senator Muir of the Australian Motoring Enthusiast Party said after consulting doctors in Victoria's Gippsland and listening to his constituents he would be very unlikely to support the government. "There is very little chance I could even consider supporting the government's proposed changes to the GP rebates," he told Fairfax Media. South Australian Senator Xenophon described it as a "knee-jerk reaction driven by bean-counters" and said he didn't know why the government wanted to be "at war" with GPs instead of working with them to drive savings through the public health system. "I'm inclined to disallow it," he told Fairfax Media on Tuesday. The Senate could "disallow" an "instrument" presented to Parliament, although 39 votes are needed, which would require the support of Labor, the Greens and four crossbenchers.

Shadow cabinet has not met and is still to formally decide a position but Labor leader Bill Shorten has criticised the policy while on the state election campaign in Queensland, predicting that under the changes "fewer Queensland doctors will be bulk-billing". "More Queenslanders will have to wait when they are sick and will end up going into the medical system later when they are sicker," he said. Labor's Health Spokeswoman Catherine King said without pre-empting the shadow cabinet's decision "Labor has always defended Medicare". "We have been very consistently opposed to the government's GP tax and are very concerned about what this new measures, this new GP tax by stealth, will mean for Australian's universal health insurance system," she said. Labor holds 25 votes in the Senate. The Greens hold 10. Greens senator Richard di Natale confirmed on Tuesday the minor party would vote against the rebate changes, meaning that if the opposition supports a disallowance, they would only need the support of one more senator.

"You don't improve our health system but paying doctors less and forcing them to charge ordinary people more," Senator di Natale said. Independent senator John Madigan said the cut to the rebate would make "life much more difficult for many people who are already struggling" but did not declare a position on which way he would vote. Palmer United's leader in the Senate Glenn Lazurus was contacted for comment. A spokesman for Health Minister Sussan Ley said the government was committed to implementing the measure and the minister would speak with senators, doctors and the broader community about the benefits of the changes. "These changes simply more-accurately reflect the time a doctor spends with their patient and encourage longer GP consultations for better health outcomes, not six minute medicine," the spokesman said.

Terry Barnes, a former adviser to Tony Abbott when he was the health minister, attacked the AMA for opposing the changes, saying doctors are waging another "self interested scare campaign" and only GPs who profit from specialising in "six-minute medicine" would fear the change. "If you're rocking up to the doctor for a script, why should that be over-rebated?" he asked. Loading "It's basically moving the focus to quality of consultations not simply quantity. In many ways it's overdue. Reality is catching up with those services," he said. Follow us on Twitter