A proposed move by the federal government to keep more official documents secret could fall foul of the constitution, a legal expert says.

According to Luke Beck, a Monash university professor of constitutional law, the Labor government’s disproportionate restriction of access to documents prepared for government ministers may contradict an implied freedom of political communication.

This makes it “ripe” for a legal challenge, he said – and former senator Rex Patrick said he was willing to back a test case.

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Under current freedom of information laws, documents can be exempt from release if they have the “dominant purpose” of going before cabinet for discussion, or to brief a minister on another document that was discussed in cabinet.

But the government has put forward a new bill that changes the rules: a document could be kept secret if the “substantial purpose” of its creation was for submission to cabinet, or merely to brief a minister “in relation to issues to be considered by the Cabinet”.

This will lead to more documents being labelled as cabinet confidential and exempt from public release, Beck said.

The amendments also go against a key recommendation by the robodebt royal commission, which found cabinet exemptions should be narrowed, not widened, he said.

“Given that the robodebt royal commission said these [exemptions] should be wound back, and given that government today operates perfectly well within the current scope of the exemption, that widening of the cabinet document exemption looks to me, especially ripe for constitutional challenge,” Beck said.

The constitutional grey area arises from whether the changes will impact the right to freedom of political communication that the high court has found is implied in the constitution.

Beck said the changes will mean documents that individuals and journalists get access to today, might not be accessible once the legislation takes affect, and would have to be justified.

“Any law that reduces the ability of people to engage in discussion on political or government matters has to be proportionate to legitimate purpose,” he said.

“And if it’s not, then the legislation will be invalid”

Beck said the government hasn’t articulated a reason to change the exemption threshold for cabinet documents, or pursued changes in what would be considered in a “proportionate way”.

“One of the tests that the high court uses to work out whether a law is proportionate to a legitimate purpose is to ask, are there any less restrictive means of achieving that purpose?” he said.

“The answer to that is the current exemption seems to work perfectly well, why do you need to widen it?”

The government has argued several reasons for the changes, including that vexatious claims and AI generated requests are clogging up the system, wasting public servants’ time and departmental resources.

The amendments have been widely criticised, including by the opposition who immediately labelled them a “truth tax”, due to the proposed introduction of an FoI application fee.

Neither the Coalition nor the Greens, who have also been heavily critical, have committed to supporting the government to pass the bill.

Patrick, the former senator turned FoI campaigner, said the current exemptions of cabinet documents were in place to protect cabinet solidarity and collective responsibility, so ministers’ differing views within the cabinet room are not made public, but Labor’s bill “goes well beyond that”.

“It makes it impossible to get access to information regarding the important matters that cabinet talks about – new laws, new policies, major projects, matters of national significance,” he said.

“It’s disgraceful that a prime minister who in opposition was so strongly in favour of transparency has tried to wrap a secrecy blanket over everything that is done at senior levels of government.”

Patrick said he would be “happy” to be an applicant to a legal challenge if the legislation passed parliament.

“Information is to democratic participation as water is to life. Most people take water for granted until there’s an absence of it.”

Patrick and Beck also raised concerns over another amendment which would make it more difficult to access documents related to cabinet “deliberations”, including advice from the public service.

Patrick said this amendment prevents the public accessing any documents of a “controversial” nature, and that documents, even if deliberative, should be made available unless contrary to the public interest.

The attorney general, Michelle Rowland, was contacted for comment.

In response, a spokesperson said “the Albanese Government has introduced legislation to strengthen the Freedom of Information Act 1982 to ensure our FoI framework works in the interests of all Australians”.