The construction union has pledged to "vigorously fight" charges of cartel conduct laid against it by the corporate watchdog.

The Australian Competition and Consumer Commission alleges the union attempted to induce the territory's steelfixing and scaffolding businesses to engage in cartel conduct.

On Thursday, the watchdog revealed it had laid charges against the ACT branch of the Construction, Forestry, Maritime, Mining and Energy Union and its secretary Jason O’Mara.

ACCC Chair Rod Sims said the union and Mr O'Mara are each accused of attempting to "induce suppliers of steelfixing services and scaffolding services to reach cartel contracts, arrangements or understandings containing cartel provisions in relation to services provided to builders in the ACT in 2012 to 2013".