The Metropolitan Fire Brigade would no longer need to stick to the minimum agreed level of 270 firefighters on duty in the city if it is allowed to adopt a proposal raised at the Fair Work Commission.

Numbers could fall by as much as 9 per cent – or 24 firefighters and eight trucks – on days that management deem to be low-risk or when city crews need to be deployed to major jobs or regional disasters.

High-ranking officers are bitterly divided between those fearing that parts of the city could be left vulnerable and others who say the current quota is too high and who favour the plan that could rein in millions of dollars lost to penalty rates.

Chief fire officer Peter Rau raised the proposal at the Fair Work Commission as an example of the powerful firefighters' union's inflexibility and refusal to change workplace rules.

As the fight continues over the MFB's bid to tear up a 2010 employment deal, which management claims gives the union too much authority, the MFB has announced it wants to introduce a seven-level strategy, whereby level four matches current minimum fire protection and levels zero to three mean reduced resources.

But assistant chief fire officer Michael Walker has criticised the plan, warning it would significantly compromise the brigade's ability to respond to city fires.

Mr Walker raised grave concerns over putting fewer firefighters on duty based on risk factors such as weather conditions, given that most house fires occur in winter.

He said the model failed to allow for major fires such as the Coode Island blaze of 1990 and the $40 million Brooklyn meatworks fire of 2001. He likened fire services to insurance.

''You would prefer to pay every second year, but you can't guarantee no fires in an off-year,'' he said.

Factors that will be considered when determining the risk rating of a certain day and the number of duty firefighters include weather forecasts, heatwaves, public events, illness pandemics, major fires, floods and other large-scale emergencies.

Mr Rau said the same daily number of firefighters – about 300 – would continue to be rostered under the new plan, but the MFB would no longer be forced to pay lucrative overtime rates to fill a shortfall in on-duty personnel to cover those who were in training or away sick.

''What we are saying is that we will continue to roster on 300-plus people, but I don't need an enterprise agreement to tell me how many people I need to run the MFB,'' he said. ''I hold the account and will be held to account if something goes wrong.''

Ken Brown, also an assistant chief fire officer, said the new proposal would take eight crucial water tankers off-line and leave Melbourne's airports at risk on days that the MFB command believed the risk was low.

He said the tankers were strategically positioned to supply water for airport firefighters at Melbourne, Essendon and Moorabbin airports to create thousands of litres of foam within the first three minutes of an emergency.

United Firefighters Union secretary Peter Marshall said the community expected the MFB to be prepared for emergencies ranging from house fires and car crashes to major disasters, as well as continuing to support other emergency services.

''The MFB proposal is a reckless gamble with the lives and well-being of Melburnians,'' Mr Marshall said.

''A delay of a few minutes can be the difference between life and death, between relatively minor damage or the loss of an entire house.''

Others in the MFB's leadership team told the commission the union alienated those who failed to ''toe the collective line'', describing a culture in which the union restricted the brigade from running effectively.