Australia's largest construction union has faced a scathing rebuke for its long record of lawlessness, which a federal judge has labelled an "embarrassment to the trade union movement".

On the eve of the federal election, the judgment lands at an uncomfortable time for Labor, of which the militant Construction, Forestry, Mining and Energy Union is a major affiliate.

The CFMEU was penalised nearly $180,000 on Friday for illegally blockading a Melbourne work site in an attempt to force building giant Grocon to cede to union demands.

Justice Christopher Jessup slammed the CFMEU's extensive history of violating workplace law.