Construction workers have rallied for a third day outside a Brisbane court to support high-profile unionist Bob Carnegie, who has been charged over his alleged involvement in a workplace dispute.

Carnegie is facing 18 charges for allegedly breaching a court order he stay away from the Children's Hospital construction site at South Brisbane during a protracted industrial dispute last year.

The Federal Magistrates Court today heard a recorded interview with Carnegie in which he said he felt he had a "moral duty to defy court orders" during the dispute.

Yesterday the court heard evidence from a private investigator hired to film Carnegie's activities at the site.

Carnegie is accused of attending the protests, organising people to picket, and using abusive language.

He has pleaded not guilty to all charges.