The Canberra Times understands there were only six paramedic crews available, when Fire and Rescue workers covered for ambulance crews.

Canberra paramedics have been working overtime to meet minimum crewing requirements, Transport Workers Union spokesperson Ben Sweaney said.

"Clearly Saturday night's situation illustrates we are at a crisis point and the government needs to commit to increasing resources across the service," he said.

"Far too often shifts are filled on overtime, often overtime that can't be filled.

"Officers are reporting the service is currently on a knife's edge.

"More needs to be done to increase numbers of paramedics and resources across the service."

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United Firefighters Union secretary Greg McConville called the incident "unprecedented".

"It's a reasonably common occurrence that fire crews assist in life threatening medical emergencies. What's different here is that an additional appliance was rostered for duty to make up for a shortfall in ambulance. To my mind, that is unprecedented," he said.

"There is an issue with minimum crewing within the ambulance service, and that needs to be resolved.

"It would be concerning if it started to shift towards that being a more common occurrence [and] in circumstances where they weren't enough paramedics available."

Mr McConville told the ABC a firefighter had to drive an ambulance after a "rather traumatic" fatal car crash at Black Mountain on Saturday.

"Quite often there are circumstances where because of inadequate crewing a firefighter will [drive an ambulance]," he said.

"We have our own crewing issues as well ... but it raises the question of what's being funded at the frontline of emergency services."

ACT Chief Fire Officer Matthew Mavity said assisting ambulance services was "part of our everyday business".

"We're trained and equipped to assist all of the other emergency services and police in the conduct of their duties," he said.

"We had no increase in activity over the weekend, we actually had a fairly quiet weekend except for the tragic incident on Black Mountain.

"There has been no increase in medical work, it just comes at different times."

Mr Mavity said assisting with ACT Ambulance did not reduce the ability of firefighters to attend to blazes.

"We had no loss of capability at all across Fire and Rescue, we were 100 percent ready," he said.

When asked what kinds of ambulance work firefighters were able to do, Mr Mavity said "you name it, we do it", adding firefighters were trained in advanced first aid trauma and medicine.

Mr Mavity also said the service operated on "medical first response", and that they attend emergencies before ambulance services about 50 times a year.

"That means if we're going to be quickest we go ... [about once] a week, which isn't a huge burden on our work," he said.

"We're always backed up by ambulance because they're the experts."

While Mr McConville from the firefighters union said there were crewing issues in ACT Fire and Rescue, Mr Mavity said "we're still the fastest fire service in Australia to respond".

He said the agency had exceeded enterprise agreement staffing numbers by ten per cent, but said the service was "very particular about the people we pick".

"I am happy with how many [staff] I have got and how well they perform," he said.

"[But] the entire employment market having a squeeze at the moment ... [staff] need to be mentally, psychologically resilient for the situations we confront. They need to be fit, they need to be smart and adaptable. And that's a pretty hard mix, quite frankly, to get.

"So we are experiencing the same difficulties as everyone else."

Mr Mavity also said there was a delay in getting some equipment on the road due to COVID-19.

"When COVID impacted on the delivery of fire trucks, and whether we can train or not in big groups, it has slowed down the delivery of those things," he said.

"At the end of August this year, we'll have our specialist combined aerial pumping appliance on the road. That's a slight delay from what we would have liked, but at the end of the day, safety is always our key thing."

"Mostly to do with how many people we can recruit, and and how many people we can promote into particular ranks, we're a bit behind. But we are still the fastest fire service in Australia to respond, so I perceived no risk to the community."

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