Legal action by the Fair Work Building Industry Inspectorate against a major trade union has been branded an "abuse of process" by the Federal Court in Brisbane.

Key points: Fair Work was "unjustifiably vexatious", Justice Rangiah rules

Fair Work was "unjustifiably vexatious", Justice Rangiah rules Court action against CFMEU deemed an abuse of process

Court action against CFMEU deemed an abuse of process Union says the case only served the Liberal Party

Fair Work took the Construction, Forestry, Mining and Energy Union (CFMEU) to court over industrial action taken at two Brisbane worksites in 2013.

It alleged the CFMEU and 21 of its organisers broke the law 822 times by shutting down the sites at the Enoggera Army Barracks and Queensland University of Technology for a combined 97 days.

Fair Work claimed the strikes were a campaign to force contractor John Holland Queensland into an enterprise agreement and were seeking up to $20 million in fines from the CFMEU.

But John Holland has already taken legal action with the union and four of its organisers over the strikes.

The parties reached a settlement.

Director of Fair Work 'unjustifiably vexatious'

Because of that, Justice Darryl Rangiah on Thursday morning ruled the director of Fair Work was being "unjustifiably vexatious" and was seeking to "relitigate" matters.

"The current proceeding is an abuse of process to the extent that it relates to the matters that were the subject of the previous proceeding," Justice Rangiah wrote in his judgment.

The finding of an abuse of process only relates to the union and the four organisers who have already been the subject of legal action.

The 17 other union organisers and delegates could potentially still face fines.

Justice Rangiah is yet to determine to what extent to the case overlaps with the previous one.

Fair Work director Nigel Hadgkiss defended the decision to pursue the matter.

"We will continue to explore all legal avenues possible to ensure that those who allegedly breach the Fair Work Act are accountable to the courts," Mr Hadgkiss said in a statement.

Case was not in public interest: union

CFMEU national construction secretary Dave Noonan said Fair Work should not be reviving settled cases.

"These abuses of process and faulty accusations are the very same cases that are being held up by [Employment Minister] Michaelia Cash and [Prime Minister] Malcolm Turnbull as justification for their draconian ABCC [Australian Building and Construction Commission] legislation," he said.

"Running this case was not in the public interest — this case only served the interests of the Liberal Party."