In short:

Some Victorian firefighters are being threatened with legal action to force them to pay a $644 levy.

The firefighters' union says the court proceedings may be needed to help them recover debt from mounting legal costs.

The legal levy is to pay for ongoing action against Fire Rescue Victoria over the the firefighters' Enterprise Bargaining Agreement.

The Victorian firefighters' union is threatening legal action against its own members for not paying a $644 levy to cover its hefty legal costs.

In what some firefighters have called a "ridiculous" move, the union last year resolved to charge all members the fee to cover the union's legal bills which financial reports show ran to almost $2.3 million last financial year.

The levy is on top of union membership fees, which are more than $2,000 per year for senior firefighters.

The letters, sent by law firm Slater and Gordon, notify members who haven't paid the levy that if the matter isn't resolved, court proceedings will be commenced against them to recover the debt.

"If proceedings are commenced, in addition to being required to pay the CCF [Court Case Funding] levy, you may also be ordered to pay interest and costs," the letter says.

One union member and senior firefighter, speaking to the ABC on the condition of anonymity, said the union should be supporting members, not attacking them.

"[It's] unhelpful and detrimental to the mental health and wellbeing of our people," he said.

He also questioned whether the cost of using a legal firm to recover the debt would actually outstrip the cost of the debt itself.

United Firefighters Union Victoria branch secretary Peter Marshall said in a statement the decision to impose the levy was made by members over five union meetings, and carried unanimously.

"Most members expected the request and have already paid," he said.

"No member received a legal letter out of the blue. Every one of them was contacted several times first.

"If there are any other members who have financial or other struggles, it is well known that I can be reached at any time, and their circumstances will be dealt with sensitivity and in confidence."

The union would not say how many letters have been sent.

Union spends more than $10m in legal fees over six years

Over the past six years, the Victorian branch of the union has spent more than $10 million on legal fees, according to its own financial statements.

The current legal levy is to pay for ongoing action the union is taking against Fire Rescue Victoria (FRV) in the Fair Work Commission over the firefighter's Enterprise Bargaining Agreement (EBA), according to firefighters the ABC spoke to.

The parties have been unable to resolve the EBA since 2021, with FRV making the union seven separate offers over that time.

The Fair Work case is just one of several legal actions the union had underway.

The United Firefighters Union of Australia and its secretary Peter Marshall also mounted Supreme court action against the Independent Broad-based Anti-corruption Commission (IBAC) this year, trying to stop the public release of a report that examined former Labor ministers and Mr Marshall.

The bid ultimately failed, with the report published in August finding Mr Marshall was able to exert extraordinary influence over the negotiations for the 2016 firefighters EBA and was given privileged access to senior government figures.

This included holding secret meetings with the then-special minister of state, Gavin Jennings, to thrash out a deal, unknown to the then Emergency Services Minister Jane Garrett, who opposed the union's demands.

At the time the IBAC report was handed down, Mr Marshall said he did not regret anything he had done to stand up for UFU members.

The union also has several other matters against Fire Rescue Victoria in the Fair Work Commission.

Several FRV employees told the ABC that the number of legal cases being pursued by the union was a point of frustration.

"If the union and its leadership had managed its money better and had not been so litigious, the members themselves wouldn't have to stump up for these additional costs," a senior firefighter told the ABC.

It's the second time the union has charged extra fees to members on top of their dues because of the union's legal bill.

Between 2014 and 2018, it charged members $500 a year to cover the union's legal costs against the Napthine Liberal government.