Internal documents reveal firefighters worked $4 million worth of extra hours in 2015-16, contributing to more than half the Emergency Service Agency ACT's entire overtime bill. It was also more than 60 per cent up from the previous period and double the 2013-14 figure, despite only a small rise in the number of firefighters.

On average, each of the 344 firefighters was paid more than $11,500 in overtime in the most recent financial year.

The figures were revealed in an internal overtime reduction plan ACT Fire & Rescue rolled out in October 2016, which acknowledged unsustainable overtime. There were not enough frontline staff to cover leave and various programs, the plan read, meaning firefighters were recalled to work to fill front-line duties and specialist positions.

The two-year plan listed 15 urgent actions aimed at fixing the problem, including recruiting 16 more firefighters, improving oversight of leave approval and ensuring the chief officer regularly reviews absences.

Longer-term initiatives included toughening up on sick leave and considering outsourcing some parts of the recruitment program to make training cheaper.

The plan stated the agency had 20 fewer operational staff than the ACT government funded - partly due to firefighters retiring, becoming unwell or taking long-term leave - which contributed to the overtime cost.

An Emergency Services Agency spokesman said the new firefighters recruited in 2016 and the actions stated in the plan were making overtime "more sustainable".

He said the plan was designed in the wake of high overtime levels from July to September. During this period, the United Firefighters Union ACT campaigned for the training of more firefighters to replace those retiring, keep response times fast and manage the workload for exhausted firefighters.

The ESA spokesman said ACT Fire & Rescue was well-resourced to meet Canberra's challenges and that it would soon employ trained firefighters from outside the territory.

"ACT Fire and Rescue is continuing to meet its performance targets within the budget allocated, delivering value for money for the community," the spokesman said.

"When firefighter numbers fall due to retirements or resignations, ACT Fire and Rescue recruits and trains additional staff."

But United Firefighters' Union ACT branch secretary Greg McConville said this was not the case.

"There has not been adequate firefighter recruitment in recent years and that is the main reason why overtime is so high," Mr McConville said.

"There were no recruits at all in 2009, 2010, 2014 and 2015, and recruitment only restarted in 2016. Around one in three of the Fire and Rescue workforce are aged over 50, and many will leave in coming months."

Mr McConville said the number of firefighters below the funded establishment had risen by five since the plan was written and would be 40 by the next fire season.

"Without adequate recruitment the overtime reduction strategy is really a piecemeal approach to treating the symptoms of a bigger problem: not enough firefighters," Mr McConville said.

"Restricting the taking of accrued leave by firefighters is just shifting the problem from one area to another."