One of Australia’s largest unions has been accused of running an illegal slush fund to bankroll a Labor faction.

Former official Brenden Sheehan has taken the Australian Services Union and several of its senior officials to court, arguing its Queensland branch has been forcing employees to contribute to a slush fund disguised as a social fund.

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Fairfax Media broke the news of Mr Sheehan's allegations in August.

He appeared briefly in the Federal Magistrates Court in Brisbane on Wednesday, claiming the union unfairly dismissed him in May because he blew the whistle on the fund.