Victorian firefighters who attended a rally against the state government while on duty will have their pay docked, with attendees accused of misusing public resources and blocking access to hospitals.

A rift between the Labor government, Fire Rescue Victoria (FRV) and the United Firefighters Union (UFU) has been deepening in recent months over a new wage deal and claims of inadequate equipment.

On Wednesday, the UFU held a rally through Melbourne's CBD, venting anger over what it said was low pay, poor conditions and dangerous obsolete equipment.

FRV and the UFU are locked in intractable bargaining hearings at the Fair Work Commission after the parties failed to strike a deal. The government had offered a 12 per cent increase over four years plus bonuses.

Fire Rescue Victoria criticises 'incredibly disappointing' protest

Firefighters rallied in FRV uniform and used some vehicles as well, prompting a memo to all staff from FRV Commissioner Gavin Freeman hitting out at the use of FRV appliances.

FRV said the industrial action was unprotected and as a result it was legally required to dock pay for those who had attended while on shift. The agency said those workers would be docked at least four hours' pay.

"We were clear in our communications that those who wished to attend but would otherwise be working needed approval for leave,'' Mr Freeman wrote.

He also hit out at members attending in FRV uniform, saying it was "incredibly disappointing" after staff were expressly told not to wear it.

In a letter to the UFU secretary, Mr Freeman said he was concerned by the UFU's actions.

"FRV is perplexed as to why the UFU would actively induce FRV staff, as UFU members, to breach their contractual obligations to FRV and potentially commit statutory offences; or condone appliances being used for non-approved purposes,'' he wrote.

He also said writing messages and posting protest material on fire trucks was not protected industrial action.

UFU hits back, calling executives 'puppets' of government

In response to the memo, the UFU issued its own bulletin saying "members should be reassured that this type of attack by senior management unequivocally fails in its endeavours."

"It is clear, beyond doubt, that Freeman and his executive are no more than puppets to the Government. They, too, will not succeed," the bulletin said.

As part of the industrial action, the UFU is using fire trucks as mobile billboards to show their message to Victorians. It is a similar tactic employed by police and paramedics.

UFU secretary Peter Marshall said FRV was attempting to suppress dissent instead of working towards improved pay and working conditions.

"Fire Rescue Victoria senior leadership went to extraordinary lengths to try to prevent the people who keep Victorians safe from meeting in public and revealing their concerns about FRV leadership's systematic disrespect for emergency services workers, disregard for public safety and wage suppression," Mr Marshall said.

"They sent one baseless legal threat after another to try to stop Victorians finding out the truth from firefighters — that half the fleet is obsolete and unsafe, that emergency services workers have not had a pay rise for four years, and that firefighters with terminal illnesses from doing their job are cast aside by senior management without a second thought."

Minister for Emergency Services Jaclyn Symes said while she supported unions' right to lawful protest she was "deeply concerned" by unprotected action.

"The UFU claims to have community safety as their highest priority, yet they put Victorians at risk by taking trucks away from the frontline to block city traffic and impede access to a hospital,'' Ms Symes said.

She said the state had invested hundreds of millions of dollars in FRV, including in this year's budget, with more than $15 million for five new FRV pumper platforms.