The arrangement was put in place at the beginning of the pandemic as a measure to deal with extreme demand to services.
As part of Covid emergency plans, Scottish firefighters were trained to drive ambulances and give first aid under the direction of a paramedic.
However, as ambulance staff face increased calls and further waits, these planned measures have now been used in the past three weeks or so to help the NHS cope.
The British Red Cross has also transported a small number of patients.
David O'Connor, regional organiser for Unison covering the Ambulance, said the use of the SFRS and other volunteers points to a ‘last resort’ and the ‘critical stage’ the service is at.
He noted it is a ‘matter of urgency’ to resolve staff welfare issues.
Mr O’Connor said: “Since Covid, there has been increased expectations put on ambulance staff.
"Because of the increased calls and further waits at A & E, ambulance staff aren’t getting breaks so we are seeing a lot of folk working beyond nine hours without a break and it’s got to the point where they can’t really function and they are not safe.
"It’s not right to ask ambulance drivers to drive a five tonne vehicle around without giving them a break – we can’t see our members pushed to this limit.”
Mr O’Connor said that SAS is currently understaffed and he hopes, if needs be, the Scottish Government steps in to ensure measures are put in place for paramedics and technicians to get appropriate breaks at appropriate times.
The issue of ‘unsustainable’ demand for ambulance staff is nationwide, according to Unison, and the trade union is in touch with politicians and chief executives to resolve this.
Mr O’Connor added: “This is the first time I’ve seen our people reach a point where they’ve had to phone their managers about their welfare about how they are not coping and they are thinking of leaving.
"Two years ago we were saying that we see ourselves as one of the best ambulance services but now we don’t feel like that anymore.”