The nation's most prestigious universities have launched a savage attack on the Turnbull government's higher education proposals, decrying them as a "contradictory, incoherent mess" developed on the "back of an envelope".

The Group of Eight (Go8) universities - representing institutions such as the University of Sydney and University of Melbourne - said it would urge the Senate crossbench to block the "fatally flawed" package in its entirety.

The Turnbull government has introduced legislation to increase university fees by 7.5 per cent, slash the HECS repayment threshold to $42,000 and cut teaching grants through a new 2.5 per cent "efficiency dividend".

University leaders across the sector have hardened their opposition to the government's package since it was announced just before the May budget.

Go8 chief executive Vicki Thomson said the package represented the biggest cuts to university funding since the Howard government came to power in 1996.

"This package, for which we were not consulted, is fatally flawed in multiple ways, and will severely harm Australia's highly successful university system," Ms Thomson said.

"In total it's a contradictory, incoherent mess.

"Our trust now lies in the Senate having a commitment to good public policy and blocking the legislation.

"We need to go back to basics, with honest data, and professional modelling of outcomes rather than back of the envelope guesswork and 'judgement calls' as the government's own education department officials have labelled the basis for some policy decisions."

The Go8 universities were the biggest supporters of the Coalition's dumped 2014 budget plan to allow universities to set their own fees.

In a letter to Senate kingmaker Nick Xenophon, Ms Thomson said the government's bill is "completely irredeemable" and must be voted down.

"The dangers arising from this legislation are profound," she said.

The University of Adelaide in Senator Xenophon's home state of South Australia would receive a $30 million funding cut over the next four years, Ms Thomson writes in the letter.

Peak lobby group Universities Australia last month announced the sector's "unanimous" opposition to funding cuts and fee hikes.

"The grim reality is that if these cuts are passed, most institutions do not have capacity to absorb them," Universities Australia chief executive Belinda Robinson said.

"At every university, students would pay more to get less.

"Enough is enough - universities and their students have already done more than their fair share for budget repair."

Education Minister Simon Birmingham said he had consulted widely on the changes and university funding was growing by 23v per cent over the next four years.

"Australian universities have seen record growth in revenue over recent years and have little to fear from slightly lower growth for a couple of years as we seek to better share the cost of a degree between taxpayers, students and universities," he said.

Labor and the Greens have announced they will vote against the government's changes, meaning it would have to win the support of the Senate crossbench to pass them into law.