Victoria's CFA insists its fire trucks are among the safest in the world and denies a design fault is putting firefighters' lives at risk.
State Opposition Leader Daniel Andrews says there is a critical flaw in a "significant" number of the tankers believed to now be out on the frontline.
He says the fault means if a burnover occurs, firefighters could be placed in a life-threatening situation.
"The safety and the security of our serving firefighters is far too important for the government to try and bury their head in the sand on this," he told reporters.
However a CFA spokeswoman said there was no design fault, and the Crew Protection System (CPS) had already saved a number of lives.
"It is a design feature that crews need to be aware of - when the CPS is activated, water automatically draws from the on-board tank," she said.
"The vast majority of cases in which CPS is used involves trucks on an active fireground, often away from permanent water sources."
The spokeswoman said every medium and large CFA tanker would be equipped with a CPS by the end of the year, making it the safest fleet in the country, if not the world.