Constitutional expert Anne Twomey has warned Bridget McKenzie that “ignorance of the law is no excuse”, chastising the former sports minister for failing to get advice on the source of her purported legal authority to give $100m in grants.

Twomey appeared before the Senate inquiry into the $100m community sport infrastructure grant program on Thursday, along with aggrieved clubs including the Beechworth Lawn Tennis Club, which has threatened to bring legal action if further grants are not made.

Although the government continues to insist the program was lawful, it has refused to release legal advice and Scott Morrison has sidestepped questions about a series of late changes to the third round of grants, which McKenzie has said were made without her knowledge.

Since the Australian National Audit Office’s scathing report, which found the program was skewed to marginal and target seats, focus has turned to its suggestion McKenzie may have lacked legal authority to make grants and what should be done to compensate clubs that missed out.

Twomey is one of a number of legal experts who have argued the program is unconstitutional – because the commonwealth lacks a power to give sports grants – and that legislation gives Sports Australia, not the sports minister, authority to make grants.

Twomey told the hearing on Thursday that unless there is a further statutory authority the commonwealth had not publicly identified, she believes McKenzie and Scott Morrison lacked authority to make decisions about sports grants.

Twomey said it was “not acceptable for the minister to say she didn’t know or was not advised” about the basis of her authority.

“Ignorance of the law is no excuse – that’s what Centrelink clients are told … ministers are well supported by staff, public servants and legal advisers. Of all people, they have no excuse.”

Asked about McKenzie’s statement that after she signed off on the final round of grants on 4 April, 2019 the changes were made on 11 April without her knowledge, Twomey replied that it was “a concern” that McKenzie “did not know staff were changing decisions about expenditure of public money without her knowledge”.

“Any minister should be aware of such things and decide the final document.”

Twomey also raised the requirement for grants to be made for proper purposes, such as to encourage participation in sport, as a further reason decisions should not be based on “political factors including whether a seat is targeted in an election”.

Twomey noted the secretary of the department of prime minister and cabinet, Phil Gaetjens’, submission that McKenzie was not “unduly influenced” by political considerations, which he said were not the “primary determining factor” in her decisions.

Twomey said she was “surprised” he had not addressed the requirement in ministerial standards that ministers must behave in a lawful manner and disinterested way, arguing the standards do not allow improper political considerations to guide decisions “at all”.

“I find his submission rather surprising, because it doesn’t grapple with what the standards say,” she said.

Twomey suggested if the arbiter of ministerial standards did not enforce them, then they “don’t have much effect at all” beyond providing guidance to ministers.

Michael Crommelin and Cheryl Saunders, both professors at the Centre for Comparative Constitutional Studies, backed Twomey’s view that the grants were both unconstitutional and made without legislative authority.

Crommelin said it seemed the message of the high court’s judgment in the chaplaincy case – that commonwealth spending requires a legislative basis – “has not been received” and called on the government to release its legal advice.

“What this affair shows us is that in the absence of clear statutory authorising a grants framework … programs of this kind can lend themselves to misuse or abuse,” Saunders said.

Andy Carr, the president of Beechworth Lawn Tennis Club, accused the government of having run a process that was “comprehensively corrupted”.

“We’re not lines in a spreadsheet, to be colour-coded and arbitrarily dismissed because of where we live or how useful we are in a political process,” he said.

Carr said applicants were “in an extremely difficult position” but his club had decided to speak up about the program because they felt “cheated”.

“We shouldn’t be here, we shouldn’t be in discussions with lawyers, journalists and senators.

“We should be in discussions with contractors, building tennis courts.”

Carr called on the government to “be open and honest about what has happened”, meet with affected clubs and “come up with a plan about what you’re going to do about it”.

McKenzie and Morrison have repeatedly defended the program on the basis that Sport Australia assessed all projects ultimately funded as eligible under the guidelines.

Morrison has suggested that clubs that missed out may get funding in a future round. Maurice Blackburn, representing Beechworth Lawn Tennis Club, has written a letter of demand calling for the government to approve a grant by 17 March or face federal court action.