The Queensland Ambulance Service recently added three new supersized ambulances and 139 "stair chairs" to help paramedics avoid injury.
Despite that, firefighters helped move obese or disabled patients more than five times a week in the ten months from July 2010 to April 2011.
The rate of assistance has jumped by one-third compared with the previous year, partly because ambulance officers were not prepared to risk injury while lifting grossly overweight patients.
In 2009, a north Queensland paramedic sued the QAS for more than $700,000 for injuries he suffered when moving a 120kg patient from a stretcher into the back of an ambulance.
Although the level of assistance provided by firefighters was a small proportion of their workload, John Oliver, of the United Firefighters Union, said it could take them away from other duties.
"It does become grating after a while. The firies' role in the community is fighting fires and rescuing people," he said.
"It really is the ambulance (officers') role. A firie will always help an ambo because that's what we do, but if it's their role, they should be doing their role."
Mr Oliver said that until recently, firefighters' main function when helping with such jobs was "clearing a path" for ambulance officers to remove obese patients, which could include knocking down doors or walls or even removing a roof for a crane-lift.
In August, 2009, firefighters organised a crane to extract a 300kg man from his apartment in Brisbane's Walter Taylor Bridge after he suffered an asthma attack and ambulance officers were unable to carry him down the stairwell.
Assisting with lifts to minimise injuries to paramedics simply "transferred the risk" to firefighters, Mr Oliver said.
"People are getting bigger, there's no doubt about it," he said.
A Department of Emergency Services spokeswoman said it was departmental policy for firefighters and ambulance officers to help each other to "ensure the best possible public safety outcome".
"The level of assistance provided by either agency is dependent on a number of factors, including other requests to respond to immediate emergency situations and the extent of training received by officers to respond to the incident," the spokeswoman said.
Jeanette Temperley, from the ambulance union United Voice, said she would raise the issue with delegates.
The union has previously raised concerns about physical injuries suffered by its members as a result of heavy lifting jobs.