The Public Service Alliance of Canada says the legislation puts people's lives in danger.
"Bill C-4 is life-threatening," says PSAC vice president Chris Aylward.
"The bill changes the definition of 'danger' to only include 'imminent' risks," Aylward told a news conference Friday.
"This means that workers will have to be in harm's way before they can establish that their working conditions are dangerous."
The new definition of danger removes the concept of complaining about unsafe work based on its impact on a worker's reproductive system, and could prevent workers from claiming they were harmed in the workplace by toxic chemicals or substances, such as asbestos, he said.
The legislation also gives the minister full authority over health and safety officers. That, says PSAC, makes it easier for employers to ignore health and safety issues in the workplace.
Once passed, the Budget Implementation Act would also give the government exclusive right to determine essential services and would limit the use of arbitration for resolving disputes.
The government is acting by "stealth," says Canadian Labour Congress president Ken Georgetti.
The amendments are "an attack on the constitutional right to collective bargaining," he said in a statement.
But it's unclear how some of the changes will affect the public service.
Treasury Board President Tony Clement said Thursday that details on how the legislation will affect public servants won't come until some time after C-4 becomes law.
Passage of the bill could happen sooner rather than later, after the Conservatives used their House of Commons majority Thursday to limit debate on the legislation.
The entire package is expected to receive a second reading vote by next week before going to a Commons committee for further scrutiny.
Many of the measures included in C-4 are related to implementing portions of the federal budget, tabled last spring.
But other sections have little, if anything, to do with budget or spending measures.
One section proposes rewriting the Supreme Court Act to declare individuals with at least 10 years on the Quebec bar at any point in their career as eligible to sit on the high court.
It's a caretaker measure aimed at giving direction to the court as it decides whether Marc Nadon is eligible to join their ranks.
Nadon was appointed to the Supreme Court by Prime Minister Stephen Harper, and later sworn in.
But the Federal Court of Appeal judge from Quebec stepped aside when a legal challenge of his appointment was launched.
C-4 would also give the immigration minister new powers to approve so-called "economic class" migrants who want to emigrate to Canada, and would extend solicitor-client privilege protections under the Proceeds of Crime (Money Laundering) and Terrorist Financing Act.
Unions see the proposes changes to labour laws included in the bill as a giant step backward for worker rights.
"These amendments, if passed, will roll back the state of labour relations 30 years by giving the employer extraordinary unchecked powers in all workplace matters," said PSAC president Robyn Benson.
"This legislation is unreasonable, and unfair."
PSAC is calling on minister Clement to meet and discuss what it sees as better ways to improve Canada's labour laws.
However, a meeting that was planned with Clement on Thursday was cancelled at the last minute.