The United Firefighters Union has taken Victoria's attorney-general to the Federal Court, accusing her of coercion and unlawful intervention.
The union's claim centres on letters Jacylyn Symes sent to Fire Rescue Victoria in August and September 2022, instructing the body not to support a new fire registration board.
The board, set up by the union, would have had complete control over who FRV could employ as an operational member.
Ms Symes, who is also the emergency services minister, told the fire service the new board had insufficient transparency and oversight measures in place.
She said FRV required her consent to sign the service agreement with the registration board and she would not provide it, the Federal Court was told on Thursday.
But union barrister Herman Borenstein KC argued FRV did not need the minister's consent and her direction was either unlawful, illegitimate or unconscionable.
"She's taken upon herself a power she didn't have," Mr Borenstein said.
The barrister directed Justice John Snaden to the Fire Rescue Victoria Act, which outlines the organisation's powers.
The act states FRV could enter into agreements or arrangements on its own volition with any person or body that's providing FRV goods or services.
The fire service only needed the relevant minister's consent if the agreement was about FRV providing another person or body with goods or services.
Mr Borenstein argued FRV was receiving the service so Ms Symes did not need to give consent, and she was unauthorised to order FRV to withdraw its support.
But solicitor-general Rowena Orr, representing the government, submitted the agreement was about FRV providing a service.
Ms Symes needed to provide her consent before FRV could do so and she was within her right to order the fire service to withdraw its support, Ms Orr said.
The solicitor-general argued even if Ms Symes was not authorised to make the direction, her actions were not unlawful, illegitimate or unconscionable.
"There's no suggestion there was any element of deliberateness," Ms Orr said.
"This is not heavy-handed ... it has none of that flavour. It has a good faith flavour."
Justice Snaden is due to hand down his judgment at a later date.
