Michael Ravbar was let go as state secretary of the Queensland CFMEU in 2024 when the controversial construction union was placed into administration nationwide.
The federal government stepped in amid claims bikies and organised crime figures had infiltrated the organisation, with Mr Ravbar and other union leaders relieved from their roles as a result.
The Fair Work Commission confirmed on Thursday it was investigating Mr Ravbar's connections with the United Firefighters' Union of Australia (UFUA).
The watchdog had received reports the UFUA had started to employ or engage Mr Ravbar, who having been removed from his position with the CFMEU is limited in which roles he can legally hold.
"The investigation will consider whether the UFUA or any of its branches have employed or engaged a person who is a 'removed person'," the commission said in a statement.
The new probe comes as a lengthy inquiry into the alleged misconduct of the Queensland CFMEU has heard allegations of stand-over tactics, harassment and bullying, and union officials operating as though they were above the law.
In June, the inquiry heard from a former senior public servant who claimed she was abused by Mr Ravbar during an "unhinged" tirade.
Andrea Fox said during a meeting with Mr Ravbar at the time she was state director of work and electrical safety policy, he threatened to have her ''physically dragged downstairs'' and thrown into the street.
''He was glaring at me and almost spitting at me … I felt in danger,'' Ms Fox told the inquiry.
Mr Ravbar denied Ms Fox's account of the interaction in a statutory declaration tendered to the inquiry, saying at no time did he yell at or threaten her.
Another ex-senior public servant told the inquiry she was sacked due to a dispute with Mr Ravbar.
Kym Bancroft, a former deputy director-general at Queensland's Office of Industrial Relations, said she was told by her boss Michael De'Ath that her position was terminated because "there is a union that's unhappy with you".
During one meeting, Ms Bancroft alleged Mr Ravbar told her she had crossed the CFMEU at her peril.
"He looked at me and said, 'Kym, you've chosen to take us on - anyone who does that does not win','' Ms Bancroft told the inquiry.
"I most definitely did not win," she continued.
"He was most definitely 100 per cent correct in his statement."
The UFUA has been contacted for comment.
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