"[We] won't go down without a fight", ACT branch secretary of the Construction, Forestry, Mining and Energy Union, Dean Hall, said on Sunday.

His strong reaction came after the federal court on Friday fined the union and five of its officials $545,000 for unlawful coercion at a housing project for the long term homeless in Brisbane.

A statement on Sunday from Fair Work Building and Construction quoted the court judgement as saying: "An industrial organisation, be it an employer organisation or an employee organisation, which persistently abuses the privilege by engaging in unlawful conduct cannot expect to remain registered."

The court referred to the CFMEU's "outrageous disregard in the past and also in the present case of Australian industrial norms".

The action was against the union's arch enemy, Grocon, at the taxpayer-funded housing project.

Mr Hall said the union was taking the threat by the federal court seriously but lashed out at the federal government which has convened a trade union royal commission.

"This is again another case that clearly shows the Fair Work Building Inspectorate is the political apparatchik of the Liberal Party," he said.

"What they're doing is prosecuting an agenda which is to remove unions from the construction industry.

"Deregistration is a very serious issue for any union to face but the real issue at play here is they're saying they're going to deregulate the construction industry and that means workers will be exposed to serious safety issues that not only can injure them but can kill them.

"In Canberra the construction industry still has the worst safety record of any jurisdiction in the country."

"The agenda is to let builders and developers have a deregulated, unprotected workforce in the construction industry without a strong union like the CFMEU.

"The end result will be workers will be killed and injured.

"Without a doubt, there's a possibility [of deregistration], when the government speaks about deregistration, you can't take it lightly but what we'll tell you right now is, the CFMEU won't go down without a fight.

"We're known for our tough position on safety and our tough position on industrial relations, we're not going to walk away and allow our members and the whole industry not to be represented.

"But there is no doubt when you have the unrestricted purse of the federal government, where they can continually run case after case, often on the most frivolous and vexatious issues to trap you and try to build up some case against you for deregistration, it is very hard.

"We're now having a fight with a federal government agency which has an unlimited bank balance."