But the government says the changes, called Work Choice, will provide greater flexibility and more jobs in small business.
Unions have begun a new wave of campaigning with television ads warning workers would not be protected from unfair dismissal.
ACTU secretary Greg Combet says the first person who's sacked unfairly, which he predicts could occur as early as Monday, can send a thank-you note to Prime Minister John Howard.
But Mr Howard says he's prepared for a union scare campaign and predicts it will fail.
Parts of the new laws took affect soon after parliament passed them late last year, but the bulk of the powers have only just come into force, including arrangements to move many state workers into the federal system.
Also under the changes companies with 100 or fewer staff will be exempt from unfair dismissal laws.
Opposition IR spokesman Stephen Smith says Labor will ensure voters are aware of the impact of the changes at the next election.
Business leader Heather Ridout says the reforms will help Australia to compete globally, but welfare groups have warned the low-paid may miss out on any benefits.