Victoria’s attorney general has launched an extraordinary attack on the integrity expert, Geoffrey Watson SC, over his claim corruption at the Construction, Forestry and Maritime Employees Union (CFMEU) cost taxpayers at least $15bn.

The figure was contained in redacted chapters from Watson’s Rotting from the Top report, commissioned by the CFMEU administrator, Mark Irving KC, and tendered during a Queensland inquiry into the union last week.

Watson, a barrister, director of the Centre for Public Integrity and former counsel assisting at the NSW Independent Commission Against Corruption, described it as a “very rough” estimate based on the opinions of “highly qualified stakeholders”.

Given Victoria’s Big Build public infrastructure program was worth roughly $100bn, and industry sources told Watson that cost blowouts linked to CFMEU conduct ranged between 10% and 30%, he settled on an estimate of 15%, describing it as “not unreasonable” and “probably conservative”.

“From there the maths is simple – the leadership of the CFMEU has cost the Victorian taxpayer something like $15bn,” the redacted chapter reads.

“There is another point to this – as will be seen, much of that $15bn has been poured directly into the hands of criminals and organised crime gangs.”

In another redacted chapter, Watson alleged the Victorian government “knew and had a duty to know” that corrupt union and underworld figures had infiltrated the Big Build but chose to do “nothing about it”.

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The chapters were removed from the final report by Irving “because he was not satisfied that they were well-founded or properly tested”, his spokesperson said.

Kilkenny said it was “reckless for anyone to make unfounded claims of a $15bn cost to taxpayers from our Big Build”.

“As lawyers, our professional credibility rests on evidence. It goes to who we are. We are bound to make a clear distinction between allegation and proof, and the rule of law depends on this,” she said in a statement on Wednesday afternoon.

“The line between fact and allegation has now been blurred. Mark Irving saw that, lawyers see that and we see that.”

Kilkenny’s intervention followed comments by the state police minister, Anthony Carbines, outside parliament on Wednesday morning, in which he accused Watson of giving “lots of florid ramblings … but not a lot of evidence”.

Carbines said if Watson had evidence to “give it to Victoria police” instead of seeking a “headline”.

Watson hit back at Kilkenny and Carbines’ comments, saying that while he has “deliberately refrained from speaking publicly” about the report the ministers’ intervention was “so inappropriate” he was obliged to respond.

“They shouldn’t have said that. It crossed a line,” Watson told Guardian Australia.

“I don’t know why they are attacking me personally rather than addressing the genuine problems in Victoria. Neither of them have bothered to call me to talk to me about it.”

The shadow attorney general, James Newbury, said the ministers “aren’t fit for their jobs and deserve to be sacked”.

“The Victorian government is so rotten that the attorney and police minister have attacked the anti-corruption expert who has exposed the worst corruption our state has ever been,” Newbury said.

“No wonder corruption thrives in Victoria.”