University students will have to pay back student loans when their earnings hit $45,000 under federal government measures to rein in bulging university costs.

Education Minister Simon Birmingham wants to cap student loans because some students are racking up excessive debt by juggling multiple university degrees.

Legislation is before the Senate to reduce the student loan repayment threshold from $55,000 to $45,000 a year.

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The bill also limits how much students can borrow from taxpayers at $104,440 or $150,000 for medicine, dentistry and vet science students.

"It's clear some students are racking up unnecessary loans that are all funded by taxpayers," Senator Birmingham told The Australian.

The government wanted to cap loans for life, but dumped the measure in favour of making the debt limit renewable.

Under the change, students will be able to borrow multiple times provided they have paid off enough debt to remain below the limit.

Greens leader Richard Di Natale said it was another example of the government making life harder for young people.

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"It is no secret that young people are getting royally screwed over by this older generation of LNP politicians," Senator Di Natale told parliament on Tuesday.

The government will have to win crossbench support for the student loan reforms with Labor and the Greens opposed.

In March, Centre Alliance, which controls two Senate votes, signalled it could support the tweaked package.

Labor senator Murray Watt said the government was paying for company tax cuts by slashing education funding.

"Right now it's students in universities who are in the gun for this government, but it really doesn't matter what type of young person you are in this country - you are in the sights of this government," Senator Watt said.

The move to cut the HECS-HELP threshold was flagged in December's mid-year budget update, with the amount lower than the $42,000 originally outlined in the 2017 budget.