The national construction industry watchdog has denied covertly spying on protesters at work site rallies, but admits it does openly snap pictures and film people at strikes.

Officers at Fair Work Building and Construction collect audio and visual evidence – sometimes on their own phones – of unlawful strikes or aggressive union behaviour.

Director Nigel Hadgkiss said the watchdog had never paid or authorised any third party to conduct surveillance work, and his officers on the job complied with all state and territory privacy laws.

“There is no covert surveillance conducted by my unit, never has been,” he told a Senate estimates hearing in Canberra on Monday.

But officers couldn’t be expected to alert everyone at a 10,000-strong protest that they could be filmed, he said.

Most rallies took place in public areas where normal laws applied.

Labor senator Doug Cameron, a former union boss, demanded to know how the watchdog could target workers “in a free country” without knowing whether the rally was illegal.

“Generally it is, senator,” Mr Hadgkiss responded.

“There is always in my experience those who attend these kind of rallies who are there unlawfully.”

Officers also film instances of assault involving union officials and hand on the footage or images to police.

Any footage not used in a case or hearing was destroyed.