The case of a Brisbane man charged with contempt of court over a series of construction site protests returns to the Federal Magistrates Court today.

Yesterday, hundreds of union supporters and construction workers rallied outside the Federal Court building to show their support for unionist Bob Carnegie.

Carnegie is facing 18 alleged breaches of a Federal Court order that banned him from picketing at a South Brisbane construction site last year.

The order was brought against 16 respondents but Carnegie, who is not a site worker or a construction union member, was the only to be charged.

Yesterday, the court heard evidence from a private investigator who allegedly filmed Carnegie at construction company Abigroup's children's hospital site in South Brisbane during the nine weeks of industrial action.

Carnegie was originally facing 54 counts of contempt.

Yesterday he was found not guilty of 34 and a further two charges were dropped.

Abigroup has declined to comment on the case.