The Turnbull government could have a battle on its hands over the new income threshold for the Medicare levy.
The government wants to increase the levy by half a percentage point to 2.5 per cent of taxable income from July 1, 2019, to ensure the national disability insurance scheme is fully funded.
In Tuesday's budget, it also increased the Medicare levy low-income thresholds for singles, families and seniors.
The new thresholds are $21,655 for singles, $36,541 for families (plus $3356 for each dependent child or student), $34,244 for single seniors and pensioners and $47,670 (plus $3356 for each dependent child or student) for senior and pensioner families.
Crossbench senator Jacqui Lambie said she was concerned about the level at which the levy kicked in, and understood Labor also had concerns.
"I think they need to start that maybe in the mid-$30,000 mark - I think starting around that $22,000 is too low," she told Sky News on Wednesday.
"These guys are (earning) just over what welfare recipients receive. I am concerned about that and I believe Labor is concerned about that as well."
South Australian senator Nick Xenophon, whose team commands three votes, shares her concerns.
"We need to make sure at the level it cuts in that it doesn't hurt the most vulnerable Australians and those on particularly low incomes," he said.
Greens leader Richard Di Natale said the income threshold should be lifted but he had wider concerns about the need for extra taxes on low-income earners.
"Why are you giving a tax break to someone on a million bucks but increasing taxes if you are on $22,000 a year?" he told reporters in Canberra.
"We will be talking to the government about how they can make that reform fairer."
Treasurer Scott Morrison said discussion about how the levy was structured was a "completely different discussion" for another time.
He said the increase was fair.
"If you earn more money you are paying a higher levy, if you earn less money you pay a lower levy," Mr Morrison told the National Press Club.
"This isn't about ideology. This isn't about setting people against each other. This is about all of us accepting that we've got a responsibility."
Australian Council of Social Service chief executive Cassandra Goldie said the government should look at changing the Medicare surcharge levy on high-income earners and winding back negative gearing and capital gains tax breaks.
But the welfare sector would not object to the Medicare levy increase to fund a "decent NDIS", which all Australians wanted.
