ACT Fire and Rescue's annual overtime bill has increased by $800,000 since the introduction of a plan designed to cut back on the payments.
The territory's urban fire service implemented an overtime reduction plan in 2016 after forking out what the plan called an "unsustainable" $4.1 million on overtime payments in 2015-16.
An Emergency Services Agency spokeswoman said ACT Fire and Rescue had again dished out $4.1 million in overtime payments in 2016-17, followed by $4.9 million last financial year.
The spokeswoman said the agency was reviewing its policies to reduce any risk of excessive overtime, and that firefighters were able to decline requests to work overtime.
In a note sent to staff on March 27 and subsequently published on the Emergency Services Agency website, ACT Fire and Rescue chief officer Mark Brown addressed what the note described as a dramatic increase in overtime hours since 2016.
"This is because there has been a higher than expected number of [ACT Fire and Rescue] staff retiring recently, as well as a number of staff who have taken long-term leave or are medically unfit for front-line duties," Mr Brown said.
"[ACT Fire and Rescue] has been recruiting additional firefighters as quickly as possible, but in the short to medium-term, vacancies will continue to need to be covered by overtime."
United Firefighters Union ACT branch secretary Greg McConville said documents obtained by the union through the Federal Court indicated the overtime bill would increase again in 2018-19.
He said 116,658 overtime hours had been paid to firefighters in 2017-18. Based on 2018-19 figures to date, that was on track to increase by 23,954 hours this fiscal year.
Mr McConville said about one-third of the territory's professional firefighters were over the age of 50 and the issue should have been foreseen.
"Every day that the sun comes up is a day that a firefighter gets older," he said.
"As a consequence, they get closer to retirement. There is nothing new or unique about this and they have known about this for years."
Mr McConville said it was true that firefighters had the right to decline overtime shifts.
"But they're a collective lot, they work in teams and they know that if they decline, someone else has to do the hours."
The union secretary has previously said ACT Fire and Rescue's personnel numbers meant a number of firefighters had regularly worked 34 hours in a 38-hour period.
He said this was unsafe and could result in adverse health impacts on firefighters, as well as increasing the risk that a firefighter would make a mistake on the job and put the public in danger.
In Mr Brown's message to staff, the ACT Fire and Rescue boss said the service continued to monitor firefighters' overtime hours.
"The service is also currently reviewing its stand-up arrangements for days with elevated fire danger to determine if the level of bushfire risk is outweighed by the risk of fatigue for firefighters recalled to duty to staff tankers and [compressed air foam system] units," Mr Brown said.
The Sunday Canberra Times has previously revealed ACT Fire and Rescue's proposal to halve the number of additional crews it is required to have working on days with a very high fire danger rating.
The number of overtime hours clocked up by firefighters is central to the union's current industrial action, through which it is calling for ACT Fire and Rescue to hire an extra 204 firefighters in the next four years.
Federal Court Justice Bernard Murphy last week ordered the ACT government to give the union several documents including the unredacted pay records of its members who worked overtime in the year ending March 8.
The case went to court because the government, in its lawyer's words, found itself "on the horns of a dilemma" because if it complied with the union's request for the payroll data, it would be contravening the Information Privacy Act.
But Justice Murphy described debate over whether to de-identify the pay records as "arid".
"I don't know how employees would ever enforce their rights in relation to underpayments because you wouldn't know who they were," he said.
Justice Murphy said if he ordered the release of the records, there would be no breach of the Act.
"So it's not really a dilemma, is it?" he said.
"... It doesn't look like there's any great harm going to be suffered by the ACT fire brigade."