Staff who make controversial or unwelcome public comments could be dismissed without notice under a proposed new workplace agreement, the National Tertiary Education Union has warned.

"The university is trying to avoid controversy," the union's Victorian secretary Colin Long said.

In a strongly worded letter to vice-chancellor Professor Glyn Davis, six leading academics from the university's law, health sciences, business and sciences faculties have criticised the plan as a restriction of their right to speak out "without fear or favour".

The academics who have already signed the letter include professors Sean Cooney, Beth Gaze, Christian Haesemeyer, Frances Separovic and associate professors Larry Abel and Susan Ainsworth.

"It is a right of students studying at the university to learn from academics who have reached their conclusions freely; and a right of society the University operates in to trust that scientific results are arrived at freely, without fear or favour," it said.

The proposed agreement no longer includes a definition of academic freedom but refers instead to the principle in a separate policy which it says it will "regard".

Academics said this policy could be changed at any time and has no legal clout.

"If academic freedom becomes a mere aspiration...it has no real force," the open letter states. "Only when academic freedom is enshrined as in a legally binding agreement, which includes the protection against summary dismissal for expression of unpopular positions, can it be said to truly afford the independence of thought that ground breaking research and challenging and fruitful teaching require."

A University of Melbourne spokeswoman said that the university was "steadfast in its commitment to the principle of academic freedom".

She said it looked forward to discussing the issue during the ongoing bargaining process.

Professor Gaze, an administrative law expert, said the changes could have a "chilling effect" by making academics cautious about what they said, particularly during recorded lectures.

"If there is something controversial that comes up then people may be reluctant to be caught on tape," she said.

Professor Gaze said she didn't know why the university was pushing ahead with the changes.

"If we want to have full inquiry in our university individuals should not be prohibited from making statements because they think their employer will not agree with them."

Professor Christian Haesemeyer said academics should be able to express controversial opinions in public without being punished by their employer.

"We are quite concerned," the mathematician said.

The current agreement, which expired at the end of June, states that academic freedom lets staff "engage in critical enquiry, intellectual discourse and public controversy without fear or favour, but does not include the right to harass, intimidate or vilify." This is not included in the proposed new agreement.

A number of Australian universities are pushing to remove academic freedom clauses from workplace agreements, according to Mr Long.

He said this was because they had become increasingly concerned about managing their brand, and many senior managers are not from academic backgrounds.

"It's hard to know why universities want to undermine the things that make them universities," he said.

The issue of academic freedom was thrust into the spotlight this week when Cambridge University Press blocked online access to hundreds of articles in China, including some covering the Tiananmen Square democracy protests of 1989. It later backed down, making the articles available.

In 2008 the University of Melbourne was accused of undermining academic freedom when it demoted the late Paul Mees, a respected transport academic following a complaint from the state government. At a public forum Dr Mees said the authors of a 2007 report on transport privatisation were "liars and frauds and should be in jail".

Similar concerns were raised about the University of Wollongong when it dismissed academic Ted Steele in 2001 after he alleged that he had been ordered to mark up student work.