But opposition Labor party leader Kim Beazley, who has struggled to find traction with voters, vowed to scrap the legislation if his party wins office in 18 months’ time, saying the laws erode job security and wages.
"This is a battle for ordinary Australian life. This is a battle for Australian families," Beazley told a crowd of cheering workers in Melbourne.
Police estimated that 80,000 people rallied in Melbourne and some 30,000 marched in western Sydney. Several thousand people also protested in smaller towns and cities throughout the country.
Howard, who has been in power for a decade, has long argued that collective bargaining by powerful unions has strangled productivity.
The same argument was championed by former British prime minister Margaret Thatcher, who set out to break Britain’s trade union movement in the 1980s.
Howard’s previous attempts at reform were blocked by the Senate.