SYDNEY - Thousands of Australian workers on Wednesday walked off job sites and joined protests against the federal government's crackdown on a major construction trade union.

Members and supporters of the Construction, Forestry and Maritime Employees Union (CFMEU) joined rallies in Sydney and Melbourne to protest against the government's decision to force the construction arm of the union into administration.

The government in August placed all branches of the construction arm of the CFMEU into administration in response to allegations of corruption and criminal infiltration.

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Workers on Wednesday walked off job sites for the second time to support the CFMEU after similar protests in August.

Speaking to protesters in Melbourne, CFMEU National Secretary Zach Smith said that the government's move would lead to worse wages and conditions for Australian workers, the Australian Broadcasting Corporation reported.

"An attack on one union is an attack on all," he said.

"Our victory is assured if we stand together and fight together and today we will be sending a very clear message across those corporate board rooms, across the halls of parliament that we're drawing a line in the sand."

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Troy Gray, secretary of the Electrical Trades Union in the Australian state of Victoria, reportedly threatened a three-day strike from next Wednesday if employers don't stop an "attack" on working conditions, according to the Guardian Australia.

Responding to Wednesday's protests, Murray Watt, Australia's minister for employment and workplace relations, said that the government is not targeting CFMEU members, union members or construction workers at all, the Age reported.

"We're doing this to ensure that their union gets back to its job, which is representing the interests of those workers," he told reporters in Canberra.

Unions previously said that they would go to court to challenge laws that allowed the government to take control of the construction union for a minimum of three years.