The Australian Council of Trade Unions is digging in against the Morrison government's "ensuring integrity" bill, which makes it easier to deregister unions and ban officials.
The latest campaign weapon is focusing on the bulk of unionists being volunteers, contrasting teacher and nurse representatives with construction union boss John Setka.
Labor claims the coalition is using the Victorian CFMMEU secretary as a "stalking horse" to ram through the laws, which would also create a public interest test for mergers.
Australian Education Union ACT president Angela Burroughs believes pointing to Mr Setka is unfair, accusing the government of deliberately painting an out-of-date picture of unionism.
"It's a cheap-shot and it totally overlooks that he's not representative of the average unionist," she told AAP.
She said the issues surrounding Mr Setka, who the ACTU has told to quit and Labor are trying to expel, had damaged the reputation of the union movement.
Labor and unions argue the average unionist is a woman in her 40s who works in aged care.
Ms Burroughs said the bill had extraordinary overreach that would subject volunteers like her to greater standards than company directors.
"It's just exasperating because there's no tangible benefit to promoting harmonious industrial relations. If anything, it's antagonising to the union movement," she said.
Industrial Relations Minister Christian Porter said the CFMMEU had been described in court as the most recidivist corporate offender in Australian history.
"As truly shocking as John Setka's record of law-breaking is, the problem inside the CFMMEU is beyond one person," he told parliament on Wednesday.
He said Mr Setka was responsible for just one per cent of the union's $16.4 million in fines, or 22 of the 2160 financial penalties.
The bill is being scrutinised by a Senate committee which will hold hearings in Canberra on Thursday and Brisbane on Friday.
Representatives from Unions ACT, National Tertiary Education and Union, and the Community and Public Sector Union will give evidence in the national capital.
Supporters of the bill, Master Builders Australia and the Housing Industry Association, will also front the committee.
In a submission to the inquiry, Master Builders said the standard of industrial conduct by some union in the construction industry was deplorable.
HIA argued there had been a long history and culture of intimidation, and lawlessness in the sector.