By Nick Bryant
BBC News, Sydney
The Australian union movement is holding a rock concert in Sydney to protest against the government's controversial industrial relations law.
Midnight Oil frontman Peter Garrett is now an opposition politician
With the federal election due later in the year, the legislation is one of the key areas of disagreement between the Liberal government and the opposition.
The rally includes some of Australia's top rock acts and organisers are hoping to pack out the venue.
Business groups say the old laws stifled growth and job creation.
Industrial relations is arguably the most contentious issue in Australian politics because it highlights the ideological differences between the Howard government and the Labour opposition.
During its 10 years in power, the government has scaled back workers' rights, allowing small firms to be exempt from unfair dismissal laws and making it easier for large companies with more than 101 employees to sack staff.
But the unions claim the new laws and guidelines - dubbed "Workchoices" and the biggest overhaul of industrial relations in the country's history - have taken away much-needed workplace protections.
They have also placed undue restrictions on collective bargaining.
This protest rally is one of the biggest held yet, with 40,000 people expected to cram the Sydney Cricket Ground for the Rockin For Rights event.
The acts include the Hoodoo Gurus, the Screaming Jets and the Whitlams.
The keynote speaker will be Labour MP Peter Garrett, the former lead singer of Midnight Oil, one of Australia's most famous bands.